sh: SH7786 clock framework rewrite.

This rewrites the SH7786 clock framework support completely. It's
reworked to provide all of the DIV4 and MSTP function clocks. This brings
it in line with the current clock framework code and lets us drop SH7786
from the list of CPUs that require legacy CPG handling.

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