target-microblaze: Convert version_mask to a CPU property

Originally the version_mask 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>