ASoC: cs4271: add support for AMUTEB=BMUTEC feature

The CS4271 has a feature to sync its analog mute flags, so one mute
circuitry can be used for both channels.

Give users access to this feature with a new DT property and a flag in
the platform data.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>