sh: reinstate clock framework rate rounding.

This was killed off by a simplification patch previously that failed to
take the cpufreq use case in to account, so reinstate the old bounding
logic. The lowest rate bounding on the other hand was broken in that it
never actually got assigned a rate and the best fit rate was instead just
getting lucky based on the ordering of the rate table, fix this up so the
code actually does what it was intended to do originally.

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