target-microblaze: Remove USE_MMU PVR checks

target-microblaze: Remove USE_MMU PVR checks

We already have a CPU property to control if a core has
an MMU or not. Remove USE_MMU PVR checks in favor of
looking at the property.

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>