clk: sunxi-ng: div: Switch to divider_round_rate

divider_round_rate_parent already evaluates changing the parent rate if
CLK_SET_RATE_PARENT is set. Now that we can do that on muxes too, let's
just use it.

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