The TAS5722 supports modifying volume in 0.25dB steps (as opposed to
0.5dB steps on the TAS5720). Introduce a custom mixer control that
allows taking advantage of this finer output volume granularity.

Also add custom getters/setters for access as the TAS5722 digital volume
controls are split over two registers.
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>