target-microblaze: Allow address sizes between 32 and 64 bits

Allow address sizes between 32 and 64 bits.

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>