board/zynqmp/kria/readme.txt: add usb start

When halting the boot process in u-boot, the 'usb start' command does not
automatically run.  Since the SD card is connected over a USB to SD card
bridge on the kd240 and kr260 boards, the readme.txt instructions should
document that the 'usb start' command needs to be manually enterred when
following the reflashing instructions.  Otherwise, the command to read the
boot.bin and u-boot.itb files from the SD card will fail.

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

board/zynqmp/kria/readme.txt

@@ -101,12 +101,14 @@ Flashing boot.bin:
 
 KD240 / KR260 Flashing Instructions:
 Flashing u-boot.itb:
+    $ usb start
     $ sf probe
     $ fatload usb 0 0x1000000 u-boot.itb
     $ sf erase 0x280000 +$filesize
     $ sf write 0x1000000 0x280000 $filesize
 
 Flashing boot.bin:
+    $ usb start
     $ sf probe
     $ fatload usb 0 0x1000000 boot.bin
     $ sf erase 0x200000 +$filesize