doc: fit: Write hex address as hex instead of int

When update_uboot.its is used directly there is syntax error for no reason.

Error report:
mkimage -f update_uboot.its boot
Error: update_uboot.its:18.12-13 syntax error
FATAL ERROR: Unable to parse input tree
mkimage: Can't read boot.tmp: Invalid argument

Fixes: 4bae9090 ("Automatic software update from TFTP server")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>