target-s390x: add a cpu_mmu_idx_to_asc function

Use constants to define the MMU indexes, and add a function to do
the reverse conversion of cpu_mmu_index.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>