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

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

The mapping between physical gain values and gpio values can be
expressed using ranges described in the gain-ranges property.

This gain-ranges property is an array of ranges.

Each range in the array is defined by the first point and last point in
the range. Those points are a pair of values, the gpios value and the
related gain (dB) value.

With that, a given range defines N possible items (from the first point
gpios value to the last point gpios value) in order to set a gain from
the first point gain value to the last point gain value.

Handle this description and the related kcontrol.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Link: https://patch.msgid.link/20260513081702.317117-15-herve.codina@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>