ASoC: simple-amplifier: gpio-audio-amp: Add support for gain-labels

ASoC: simple-amplifier: gpio-audio-amp: Add support for gain-labels

The possible gain values can be described using labels instead of gain
values in dB.

Those different labels are attached to a gpio values using the
gain-labels property.

Using the gain-labels description is mutually exclusive with gain-ranges
description used to describe the relationship between gpios values and
gain values.

Handle the gain-labels description and the related kcontrol.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://patch.msgid.link/20260513081702.317117-16-herve.codina@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>