clk: sunxi-ng: mux: Allow muxes to have keys

The muxes in the RTC can only be updated when setting a key field to a
specific value. Add a feature flag to denote muxes with this property.

Since so far the key value is always the same, it does not need to be
provided separately for each mux.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220203021736.13434-5-samuel@sholland.org