clk: sunxi-ng: Add fractional lib

Some clocks can be switched to a mode called fractional that have two fixed
output rate you can choose from.

Add a small library to deal with those clocks.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160629190535.11855-4-maxime.ripard@free-electrons.com