sh: clkfwk: Abstract rate rounding helper.

Presently the only assisted rate rounding is frequency table backed, but
there are cases where it's impractical to use a frequency table for
certain clocks (such as the FSIDIV case, which supports 65535 divisors),
and we wish to reuse the same rate rounding algorithm.

This breaks out the core of the rate rounding logic in to its own helper
routine and shuffles the frequency table logic around, switching to using
an iterator for the generic helper routine.

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