sh: clkfwk: Add a helper for rate rounding by divisor ranges.

This adds a new clk_rate_div_range_round() for implementing rate rounding
by divisor ranges. This can be used trivially by clocks that support
arbitrary ranged divisors without the need for rate table construction.

This should only be used by clocks that both have large divisor ranges in
addition to clocks that will never be arbitrarily scaled, as the lack of
a backing frequency table will prevent cpufreq from being able to do much
of anything with them.

Primarily intended for use as a ->recalc helper.

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