clk: sunxi-ng: Add mux clock support

Some clocks in the Allwinner SoCs clocks unit are just muxes.

However, those muxes might also be found in some other complicated clocks
that would benefit from the code in there to deal with "advanced" features,
like pre-dividers.

Introduce a set of helpers to reduce the code duplication in such cases.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160629190535.11855-6-maxime.ripard@free-electrons.com