sh: nommu: use 32-bit phys mode.

The nommu code has regressed somewhat in that 29BIT gets set for the
SH-2/2A configs regardless of the fact that they are really 32BIT sans
MMU or PMB. This does a bit of tidying to get nommu properly selecting
32BIT as it was before.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>