arm64: zynqmp: Add support for OF_SEPARATE with board DTB

OF_BOARD and OF_SEPARATE can use board specific board_fdt_blob_setup().

OF_BOARD option is mostly used for picking up DTB from certain location.

OF_SEPARATE option is used when DTB is appended after u-boot binary.

This board specific function is aligned with current version in
lib/fdtdec.c with checking CONFIG_XILINX_OF_BOARD_DTB_ADDR address first.

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