target-microblaze: Convert use-mmu to a CPU property

Originally the use-mmu PVR bits were manually set for each machine. This
is a hassle and difficult to read, instead set them based on the CPU
properties.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>