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

This patch adds support for the ADAU7002 PDM-to-I2S/TDM converter. 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
programmable control interface and the driver simply describes the static
capabilities of the chip.

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