doc: renesas: add Renesas board docs

As a starting point, list all currently supported Renesas boards.

For the RZ/N1 board, add details about booting and flashing.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>

doc/board/index.rst

@@ -34,6 +34,7 @@ Board-specific doc
    openpiton/index
    purism/index
    qualcomm/index
+   renesas/index
    rockchip/index
    samsung/index
    siemens/index

doc/board/renesas/index.rst

@@ -0,0 +1,10 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Renesas
+=======
+
+.. toctree::
+   :maxdepth: 2
+
+   renesas
+   rzn1

doc/board/renesas/renesas.rst

@@ -0,0 +1,45 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Renesas
+=======
+
+About this
+----------
+
+This document describes the information about Renesas supported boards
+and their usage steps.
+
+Renesas boards
+--------------
+
+Renesas is a SoC solutions provider for automotive and industrial applications.
+
+U-Boot supports several Renesas SoC families:
+
+* R-Car Gen2 (32-bit)
+    - Blanche board
+    - Gose board
+    - Koelsch board
+    - Lager board
+    - Silk board
+    - Porter board
+    - Stout board
+* R-Car Gen3 (64-bit)
+    - Condor board
+    - Draak board
+    - Eagle board
+    - Ebisu board
+    - Salvator-X and Salvator-XS boards
+    - ULCB board
+* R-Car Gen4 (64-bit)
+    - Falcon board
+    - Spider board
+    - Whitehawk board
+* RZ/A1 (32-bit)
+    - GR-PEACH board
+* RZ/G
+    - Beacon-rzg2 board
+    - Hihope-rzg2 board
+    - ek874 board
+* RZ/N1 (32-bit)
+    - Schneider rzn1-snarc board

doc/board/renesas/rzn1.rst

@@ -0,0 +1,76 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Renesas RZ/N1
+=============
+
+Building
+--------
+
+This document describes how to build and flash U-Boot for the RZ/N1.
+
+U-Boot
+^^^^^^
+
+Clone the U-Boot repository and build it as follows:
+
+.. code-block:: bash
+
+        git clone --depth 1 https://source.denx.de/u-boot/u-boot.git
+        cd u-boot
+        make rzn1_snarc_defconfig
+        make CROSS_COMPILE=arm-linux-gnu-
+
+This produces `u-boot` which is an ELF executable, suitable for use with `gdb`
+and JTAG debugging tools.
+
+It also produceds `u-boot.bin` which is a raw binary.
+
+Binman
+^^^^^^
+
+The BootROM in the RZ/N1 SoC expects to find the boot image in SPKG format.
+This format is documented in Chapter 7.4 of the RZ/N1 User Manual.
+
+The `binman` tool may be used to generate the SPKG format for booting.
+See tools/binman/binman.rst for details on this tool and its pre-requisites.
+
+.. code-block:: bash
+
+    binman -d arch/arm/dts/r9a06g032-rzn1-snarc.dtb -o <OUT>
+
+This will produce `u-boot.bin.spkg` in the specified <OUT> directory. It can
+then be flashed into QSPI, NAND, or loaded via USB-DFU mode.
+
+SPKG image
+^^^^^^^^^^
+
+Alternatively, the same SPKG image can be built by calling `mkimage` as follows:
+
+.. code-block:: bash
+
+    tools/mkimage -n board/schneider/rzn1-snarc/spkgimage.cfg \
+            -T spkgimage -a 0x20040000 -e 0x20040000 \
+            -d u-boot.bin u-boot.bin.spkg
+
+This produces `u-boot.bin.spkg` which can be flashed into QSPI, NAND, or loaded
+via USB-DFU mode.
+
+Take note of the load and execution address, which are encoded into the SPKG
+headers. For development convenience, mkimage computes the execution offset
+(part of the SPKG header) by subtracting the supplied load address from the
+supplied execution address.
+
+Also note there are other parameters, notably ECC configuration in the case of
+boot from NAND, specified in the `spkgimage.cfg` configuration file.
+
+Flashing
+--------
+
+The RZ/N1 is able to boot from QSPI, NAND, or via USB (DFU). In all cases the
+on-board BootROM expects for the binary to be wrapped with a "SPKG" header.
+
+It is possible to recover a bricked unit by using the USB (DFU) boot mode. This
+allows uploading U-Boot into the internal RAM. Thereafter U-Boot can be used to
+program the QSPI and/or NAND, making use of U-Boot dfu mode.
+
+Otherwise the only other option for recovery is via JTAG.