microblaze: Fix lmb memory initialization

Microblaze as Arm is using multiple memory banks which are read from DT
that's why there is a need to initialized LMB based on bd->bi_dram[].
Without this fix memory base/size is all the time 0 and image relocation is
not possible.

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