sh: cpufreq: clock framework support.

This gets the SH cpufreq working again. We follow the changes
in the AVR32 implementation for wrapping in to the clock framework.
CPUs that wish to use this are required to define rate rounding
primitives in order to satisfy clk_round_rate().

This works well enough for the common case, though we should
look at unifying this driver across all of the platforms that
implement clock framework support in one capacity or another.

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