ASoC: add TA5720 digital amplifier DT bindings

The Texas Instruments TAS5720L/M device is a high-efficiency mono
Class-D audio power amplifier optimized for high transient power
capability to use the dynamic power headroom of small loudspeakers.
Its digital time division multiplexed (TDM) interface enables up to
16 devices to share the same bus.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>