ASoC: pcm3168a: Document optional RST gpio

On boards where the RST line is not pulled up, but it is connected to a
GPIO line this property must present in order to be able to enable the
codec.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191113124734.27984-2-peter.ujfalusi@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>