sh: Migrate SH-4 cacheflush ops to function pointers.

This paves the way for allowing individual CPUs to overload the
individual flushing routines that they care about without having to
depend on weak aliases. SH-4 is converted over initially, as it wires
up pretty much everything. The majority of the other CPUs will simply use
the default no-op implementation with their own region flushers wired up.

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