sh: clkfwk: add clk_rate_mult_range_round()

This provides a clk_rate_mult_range_round() helper for use by some of the
CPG PLL ranged multipliers, following the same approach as used by the
div ranges.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>