sh: More I/O routine overhauling.

This tidies up a lot of the PIO/MMIO split. No in-tree platforms were
making use of the MMIO overloading through the machvec (nor have any of
them been in some time), so we just kill all of that off. The ISA I/O
routine wrapping remains unaffected, which remains the only special
casing outside of the iomap API that boards need to think about.

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