clk: sunxi-ng: Adjust MP clock parent rate when allowed

Currently MP clocks don't consider adjusting parent rate even if they
are allowed to do so. Such behaviour considerably lowers amount of
possible rates, which is very inconvenient when such clock is used for
pixel clock, for example.

In order to improve the situation, adjusting parent rate is considered
when allowed.

This code is inspired by clk_divider_bestdiv() function, which does
basically the same thing for different clock type.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>