xilinx: common: Do not touch CONFIG_XILINX_OF_BOARD_DTB_ADDR in SPL

This hook is used in full U-Boot that's why there is no reason to touch
this location from SPL. The hook was introduced for QEMU usage but none is
really running SPL on QEMU that's why it shouldn't break any usecase.

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