ASoC: tas6424: Add support for TAS6424 digital amplifier

The Texas Instruments TAS6424 device is a high-efficiency quad-channel
Class-D audio power amplifier. Its digital time division multiplexed
(TDM) interface enables up to 2 devices to share the same bus,
supporting a total of eight channels from one audio serial port.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Michael Stecklein <m-stecklein@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>