clk: sunxi-ng: mux: Re-adjust parent rate

Currently, the parent rate given back to the clock framework in our
request is the original parent rate we calculated before trying to round
the rate of our clock.

This works fine unless our clock also changes its parent rate, in which
case we will simply ignore that change and still use the previous parent
rate.

Create a new function to re-adjust the parent rate to take the pre-dividers
into account, and give that back to the clock framework.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>