clk: sunxi-ng: Add helper function to find closest rate

The default behaviour of clocks in the sunxi-ng driver is to select a
clock rate that is closest to but less than the requested rate.

Add the ccu_is_better_rate() helper function that - depending on the
fact if thc CCU_FEATURE_CLOSEST_RATE flag is set - decides if a rate is
closer than another rate.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Frank Oltmanns <frank@oltmanns.dev>
Link: https://lore.kernel.org/r/20230807-pll-mipi_set_rate_parent-v6-5-f173239a4b59@oltmanns.dev
Signed-off-by: Chen-Yu Tsai <wens@csie.org>