target-microblaze: mmu: Add a configurable output address mask

target-microblaze: mmu: Add a configurable output address mask

Add a configurable output address mask, used to mimic the
configurable physical address bit width.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>