microblaze: Wire CMA allocator

Based on commit 04e3543e

 ("microblaze: use the generic dma coherent
remap allocator")
CMA can be easily enabled by calling dma_contiguous_reserve() at the end of
mmu_init(). High limit is end of lowmem space which is completely unused at
this point of time.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>