sh: Isolate uncached mapping support.

This splits out the uncached mapping support under its own config option,
presently only used by 29-bit mode and 32-bit + PMB. This will make it
possible to optionally add an uncached mapping on sh64 as well as booting
without an uncached mapping for 32-bit.

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