clk: sunxi-ng: Add N-K-factor clock support

Introduce support for clocks that use a combination of two linear
multipliers.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160629190535.11855-10-maxime.ripard@free-electrons.com