ASoC: codecs: Add Awinic AW8738 audio amplifier driver

The Awinic AW8738 is a simple audio amplifier using a single GPIO.
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).

Add a simple driver that allows setting it up in the device tree
with a specified mode number.

Signed-off-by: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Co-developed-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20220304102452.26856-3-stephan@gerhold.net
Signed-off-by: Mark Brown <broonie@kernel.org>