ASoC: tas571x: new chip added into TAS571x binding

This adds the TAS5721 into the TAS571x binding.

Signed-off-by: Petr Kulhavy <petr@barix.com>
Reviewed-by: Kevin Cernekee <cernekee@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>