clk: sunxi-ng: Remove the use of rational computations

While the rational library works great, it doesn't really allow us to add
more constraints, like the minimum.

Remove that in order to be able to deal with the constraints we'll need.

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