ASoC: dt-bindings: Make RK3328 codec GPIO explicit

Existing RK3328 codec drivers have overloaded the GRF phandle to assume
implicit control of the limited-function GPIO_MUTE pin, which is usually
used to enable an external audio line driver IC. Since this pin has a
proper binding of its own (see gpio/rockchip,rk3328-grf-gpio.txt), make
a GPIO explicit in the codec binding too. This will help avoid ambiguity
on boards that use that pin for some other purpose.

(and while touching the example, enforce the "don't include status" rule)

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/5f7a399dea8a9dedef57f6f99f0f6ab1c1fdc56a.1581376744.git.robin.murphy@arm.com
Signed-off-by: Mark Brown <broonie@kernel.org>