target-microblaze: cpu_mmu_index: Fixup indentation

Fixup the indentation of cpu_mmu_index in preparation for
future edits.
No functional changes.

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>