board/zynqmp: add custom board documentation

The readme.txt file for zynqmp boards does not have documentation for working
with custom boards.  This patch adds documentation for custom board support.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/zynqmp/readme.txt

@@ -65,6 +65,47 @@ Where 'sdX' is the device node of the SD.
 
 Eject the SD card, insert it in the board, and power it up.
 
+Support for other boards:
+=========================
+
+If you want to build a system for other boards based on the same SoC, and the
+board is already supported by the upstream kernel and U-Boot, you simply need
+to change the following Buildroot options:
+
+ - Kernel Device Tree file name (BR2_LINUX_KERNEL_INTREE_DTS_NAME)
+ - U-Boot (BR2_TARGET_UBOOT_CUSTOM_MAKEOPTS="DEVICE_TREE=<dts file name>")
+
+Custom psu_init_gpl.c/h support:
+
+To generate a working boot.bin image, psu_init_gpl.c/h are required in
+the U-Boot source tree. Without those files, boot.bin will be built
+successfully but it will not be functional at all. Those files are
+output from the Xilinx tools, but for convenience, U-Boot includes the
+default psu_init_gpl.c/h of popular boards. Those files may need to be
+updated for any programmable logic or DDR customizations which impact
+psu_init (clock/pin setup & mapping/AXI bridge setup/etc). See
+board/xilinx/zynqmp/ directory of U-Boot for natively supported psu_init
+files. If the psu_init files for your board are not found in U-Boot,
+you need to add them by yourself.
+
+1) Start with a defconfig supported by Buildroot (e.g. ZCU106)
+    make zynqmp_zcu106_defconfig
+
+2) make uboot-menuconfig
+    Visit the following menu and enable CONFIG_XILINX_PS_INIT_FILE
+
+    ARM architecture  --->
+       [*] Zynq/ZynqMP PS init file(s) location
+
+3) Copy psu_init_gpl.c/h generated by the Xilinx Vivado tools into
+   output/build/uboot-custom/board/xilinx/zynqmp/custom_hw_platform/
+
+4) make
+
+Note: The files in step 3 will need to be re-copied after cleaning the
+output directory and at this time, there is no way to save them as
+part of the buildroot configuration, except as a U-Boot patch.
+
 ==============
 Important Note
 ==============