ASoC: sta350: Add codec driver

The TI STA350 is an integrated 2.1-channel power amplifier that is
controllable over I2C. This patch adds an ASoC driver for it.

At a glance, this chip is very similar to the STA320 for which a driver
already exists. In details, however, the register maps contain subtle
differences which made a whole new driver easier to write and maintain.

[daniel@zonque.org: cleanups, DT property rework, rebased on asoc-next]
Signed-off-by: Sven Brandau <brandau@gmx.de>
Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Mark Brown <broonie@linaro.org>