ASoC: rt1308-sdw: add rt1308 SdW amplifier driver

This is the initial amplifier driver for rt1308-sdw.

Signed-off-by: Shuming Fan <shumingf@realtek.com>
Link: https://lore.kernel.org/r/20200110014606.17333-1-shumingf@realtek.com


Tested-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>