board: xilinx: Enable changing default DTB pick up address

board: xilinx: Enable changing default DTB pick up address

U-Boot on xilinx boards is checking one address where DTB can be placed as
the first location for DTB. Originally this code was developed for Versal
where QEMU was putting generated DTB for U-Boot to use.
The patch enables changing this address which is necessary for cases where
default address is pointing to location (DDR) which is not present. The
access to this location can cause exception.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>