clk: sunxi: make factors clock mux mask configurable

Some of the factors-style clocks on the A80 have different widths
for the mux values in the registers.

Add a .muxmask field to clk_factors_config to make it configurable.
Passing a bitmask instead of a width parameter will allow reuse
in case we support table-based muxes in the future.

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