ASoC: Add ADAU1977 driver

This patch adds support for the ADAU1977, ADAU1978 and ADAU1979 audio CODEC
devices. They are a family of 4-channel differntial input audio ADC devices.
They can be connected to either a SPI or I2C bus.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>