xilinx: Kconfig: add XILINX_OF_BOARD_DTB_ADDR default value for microblaze

The xilinx board_fdt_blob_setup() implementation makes use of
XILINX_OF_BOARD_DTB_ADDR, but no default value is currently defined for
microblaze. Add one so that microblaze could also work with
CONFIG_OF_SEPARATE.

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Link: https://lore.kernel.org/r/20211202195657.246723-1-ovidiu.panait@windriver.com


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