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>

target/microblaze/mmu.h

@@ -72,6 +72,7 @@ struct microblaze_mmu
     int c_mmu;
     int c_mmu_tlb_access;
     int c_mmu_zones;
+    uint64_t c_addr_mask; /* Mask to apply to physical addresses.  */
 };
 
 struct microblaze_mmu_lookup