microblaze: Setup initrd_high and fdt_high at run time

Setup initrd_high and fdt_high to be placed in lowmem space for kernel to
be able to reach it. Values are setup at run time to ensure that the same
setting can be used on different memory setup. Do this setting only when
variables are not

Similar run time detection was done for Zynqmp and Versal.

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