clk: sunxi: Remove custom phase function

Now that we don't have any user left for our custom phase function, we can
safely remove this hack from the code.

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