ASoC: codecs: Add Awinic AW8738 audio amplifier driver

Merge series from Stephan Gerhold <stephan@gerhold.net>:

This series adds a simple driver and DT schema for the Awinic AW8738
audio amplifier. It's fairly simple - the main difference to
simple-amplifier is that there is a "one-wire pulse control" that
allows configuring the amplifier to one of a few pre-defined modes.
This can be used to configure the speaker-guard function (primarily
the power limit for the amplifier).