arm64: xilinx: Enable generic of_board_dtb

Modify the configuration naming to be generic to xilinx rather than
specific to Versal. The offset value is different for Zynq and ZynqMP
to avoid overlapping with FSBL.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>