am335x_evm: Update README for customization

As this is a reference platform, update the README to note which IP
blocks are required for use due to design choices of the reference
rather than required by the SoC itself.

Signed-off-by: Tom Rini <trini@ti.com>

board/ti/am335x/README

@@ -13,7 +13,33 @@ documented in TI's reference designs:
 - AM335x EVM SK
 - Beaglebone White
 - Beaglebone Black
-'
+
+Customization
+=============
+
+Given that all of the above boards are reference platforms (and the
+Beaglebone platforms are OSHA), it is likely that this platform code and
+configuration will be used as the basis of a custom platform.  It is
+worth noting that aside from things such as NAND or MMC only being
+required if a custom platform makes use of these blocks, the following
+are required, depending on design:
+
+- GPIO is only required if DDR3 power is controlled in a way similar to
+  EVM SK
+- SPI is only required for SPI flash, or exposing the SPI bus.
+
+The following blocks are required:
+- I2C, to talk with the PMIC and ensure that we do not run afoul of
+  errata 1.0.24.
+
+When removing options as part of customization,
+CONFIG_EXTRA_ENV_SETTINGS will need additional care to update for your
+needs and to remove no longer relevant options as in some cases we
+define additional text blocks (such as for NAND or DFU strings).  Also
+note that all of the SPL options are grouped together, rather than with
+the IP blocks, so both areas will need their choices updated to reflect
+the custom design.
+
 NAND
 ====