board/zynqmp/kria/readme.txt: improve documentation for qspi programming

This patch improves the documentation for kria k26 som qspi programming.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
[Peter: drop trailing spaces]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/zynqmp/kria/readme.txt

@@ -56,23 +56,39 @@ Where 'sdX' is the device node of the SD.
 
 Eject the SD card, insert it in the board, and power it up.
 
-How to write the boot.bn to QSPI boot flash
-===========================================
+How to write boot.bin and u-boot.itb to QSPI boot flash
+=======================================================
 
 The Kria SOMs are preconfigured to boot initially from QSPI.
 This makes these boards different from other ZynqMP boards
-in that the boot.bin needs to be flashed into the QSPI boot
-flash such that the U-Boot SPL can then load all of the
-remaining images from the SD card.
+in that the boot.bin and u-boot.itb files need to be flashed
+into the QSPI boot flash such that U-Boot can then load all
+of the remaining images from the SD card.
 
 In addition, the KV260 Starter Kit QSPI comes pre-flashed with
 a utility designed to make updating the QSPI flash memory
 easier.
 
-Instructions for using these utilities to update the boot.bin
+Instructions for using these utilities to update the files
 in QSPI flash can be found on the wiki link below.
 
 https://xilinx-wiki.atlassian.net/wiki/spaces/A/pages/1641152513/Kria+K26+SOM#Boot-Firmware-Updates
 
+Additionally, it is possible to use u-boot for updating the
+QSPI with new boot.bin and u-boot.itb images with the u-boot
+commands below:
+
+Flashing u-boot.itb:
+    $ sf probe
+    $ fatload mmc 1 0x1000000 u-boot.itb
+    $ sf erase 0xf80000 +$filesize
+    $ sf write 0x1000000 0xf80000 $filesize
+
+Flashing boot.bin:
+    $ sf probe
+    $ fatload mmc 1 0x1000000 boot.bin
+    $ sf erase 0x200000 +$filesize
+    $ sf write 0x1000000 0x200000 $filesize
+
 It is possible to boot the Buildroot generated SD card image without
 updating the QSPI boot.bin image, so this is an optional step.