doc: board: phytec: add phycore_imx8mp

Add documentation on how to build a bootable U-Boot image for the PHYTEC
phyCORE-i.MX 8M Plus.

Signed-off-by: Yannic Moog <y.moog@phytec.de>

doc/board/phytec/index.rst

@@ -7,3 +7,4 @@ PHYTEC
    :maxdepth: 2
 
    phycore-imx8mm
+   phycore-imx8mp

doc/board/phytec/phycore-imx8mp.rst

@@ -0,0 +1,60 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+phyCORE-i.MX 8M Plus
+====================
+
+The phyCORE-i.MX 8M Plus with 2GB of main memory is supported.
+
+Quick Start
+-----------
+
+- Build the ARM Trusted firmware binary
+- Get ddr firmware
+- Build U-Boot
+- Boot
+
+Build the ARM Trusted firmware binary
+-------------------------------------
+
+.. code-block:: bash
+
+   $ git clone https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git
+   $ cd trusted-firmware-a
+   $ export CROSS_COMPILE=aarch64-linux-gnu
+   $ export IMX_BOOT_UART_BASE=0x30860000
+   $ make PLAT=imx8mp bl31
+
+Get the ddr firmware
+--------------------
+
+.. code-block:: bash
+
+   $ wget https://www.nxp.com/lgfiles/NMG/MAD/YOCTO/firmware-imx-8.19.bin
+   $ chmod +x firmware-imx-8.19.bin
+   $ ./firmware-imx-8.19.bin
+
+Build U-Boot for SD card
+------------------------
+
+Copy binaries
+^^^^^^^^^^^^^
+
+.. code-block:: bash
+
+   $ cp <TF-A dir>/build/imx8mp/release/bl31.bin .
+   $ cp firmware-imx-8.19/firmware/ddr/synopsys/lpddr4*.bin .
+
+Build U-Boot
+^^^^^^^^^^^^
+
+.. code-block:: bash
+
+   $ make phycore-imx8mp_defconfig
+   $ make flash.bin
+
+Flash SD card
+^^^^^^^^^^^^^
+
+.. code-block:: bash
+
+   $ sudo dd if=flash.bin of=/dev/sd[x] bs=1024 seek=32 conv=sync