sh: add shared clock framework frequency table code

Add SuperH-specific clock framework helper functions:
- clk_rate_table_build() - build cpufreq table from divisors/multipliers
- clk_rate_table_round() - use cpufreq table to find matching frequency

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>