clk: sunxi-ng: mux: Support finding closest rate

When finding the best rate for a mux clock, consider rates that are
higher than the requested rate when CCU_FEATURE_ROUND_CLOSEST is used.
Furthermore, introduce an initialization macro that sets this flag.

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-v6-8-f173239a4b59@oltmanns.dev
Signed-off-by: Chen-Yu Tsai <wens@csie.org>