ASoC: dt-bindings: rockchip: Document pinctrl-names for i2s

This patch documents pinctrl-names for i2s.

Signed-off-by: Judy Hsiao <judyhsiao@chromium.org>
Link: https://lore.kernel.org/r/20220619095324.492678-4-judyhsiao@chromium.org


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