clk: max77686: Provide .recalc_rate() operation

It is usually nice to know frequency of a clock, so this patch adds a
.recalc_rate() callback returning rates of provided clocks.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>