ASoC: Add ADAU7002 Stereo PDM-to-I2S/TDM Converter DT bindings

The ADAU7002 takes a stereo PDM signal (e.g. from two digital microphones)
and converts it into a I2S/TDM PCM stream. The chip does not have a
control interface and has a single power supply that is modeled in the
devicetree.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>