target-microblaze: Use TCGv for load/store addresses

target-microblaze: Use TCGv for load/store addresses

Use TCGv for load/store addresses, allowing for future
computation of 64-bit load/store address.

No functional change.

Acked-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>