target-sparc: Add MMU_PHYS_IDX

It's handy to have a mmu idx for physical addresses, so
that mmu disabled and physical access asis can use the
same path as normal accesses.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>