sh: Use uncached I/O helpers in PMB setup.

The PMB code is an example of something that spends an absurd amount of
time running uncached when only a couple of operations really need to be.
This switches over to the shiny new uncached helpers, permitting us to
spend far more time running cached.

Additionally, MMUCR twiddling is perfectly safe from cached space given
that it's paired with a control register barrier, so fix that up, too.

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