sh: initial PMB mapping iteration by helper macro.

All of the cached/uncached mapping setup is duplicated for each size, and
also misses out on the 16MB case. Rather than duplicating the same iter
code for that we just consolidate it in to a helper macro that builds an
iter for each size. The 16MB case is then trivially bolted on at the end.

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