clk: sunxi-ng: mux: split out the pre-divider computation code

The pre-divider retrieval code was merged into the function to apply the
current pre-divider onto the parent clock rate so that we can use that
adjusted value to do our factors computation.

However, since we'll need to do the reverse operation, we need to split out
that code into a function that will be shared.

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