clk: sunxi: factors: Drop round_rate from clk ops

The common clock framework requires either determine_rate or round_rate
to be implemented. We use determine_rate so we can pass the parent index
to the get_factors callback. This cannot be done easily with round_rate,
so just drop it.

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