microblaze: Add support for reserved memory defined by DT

In DT reserved memory nodes can be added as is described by
Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
but Microblaze didn't have a support for it. The similar change was done
for ARC by commit 1b10cb21

 ("ARC: add support for reserved memory
defined by device tree"). It is pretty much enough to call
early_init_fdt_scan_reserved_mem().
Microblaze is not using initial_boot_params that's why there is no need to
call early_init_fdt_reserve_self().

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/da4395776bf0de7207767abcc8a5df05bf411816.1645795048.git.michal.simek@xilinx.com