ASoC: dt-bindings: rockchip: i2s-tdm: Make grf property optional

Turns out most things don't require the GRF, so making the property
optional is the obvious move.

Signed-off-by: Nicolas Frattaroli <frattaroli.nicolas@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221025124132.399729-2-frattaroli.nicolas@gmail.com


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