ASoC: rockchip: Make RK3328 GPIO_MUTE control explicit

The RK3328 reference design uses an external line driver IC as a buffer
on the analog codec output, enabled by the GPIO_MUTE pin, and such a
configuration is currently assumed in the codec driver's direct poking
of GRF_SOC_CON10 to control the GPIO_MUTE output value. However, some
boards wire up analog audio yet use that pin for some other purpose, so
that assumption doesn't always hold. Update this functionality to rely
on an explicit GPIO descriptor, such that it can be managed at the
board level.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/5bc383ed1832f0f5d1dcb3c97ad92fd68e5217e3.1581376744.git.robin.murphy@arm.com
Signed-off-by: Mark Brown <broonie@kernel.org>