ASoC: mediatek: mt8188: Treat DMIC_GAINx_CUR as non-volatile

The DMIC_GAINx_CUR registers contain the current (as in present) gain of
each DMIC. During capture, this gain will ramp up until a target value
is reached, and therefore the register is volatile since it is updated
automatically by hardware.

However, after capture the register's value returns to the value that
was written to it. So reading these registers returns the current gain,
and writing configures the initial gain for every capture.

>From an audio configuration perspective, reading the instantaneous gain
is not really useful. Instead, reading back the initial gain that was
configured is the desired behavior. For that reason, consider the
DMIC_GAINx_CUR registers as non-volatile, so the regmap's cache can be
used to retrieve the values, rather than requiring pm runtime resuming
the device.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20250225-genio700-dmic-v2-3-3076f5b50ef7@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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