ASoC: rockchip: i2s_tdm: Make the grf property optional

Only IO Multiplex and two TRCM modes need access to the GRF, so
making it a hard requirement is not a wise idea, as it complicates
support for newer SoCs which do not do these things.

Signed-off-by: Nicolas Frattaroli <frattaroli.nicolas@gmail.com>
Link: https://lore.kernel.org/r/20221025124132.399729-3-frattaroli.nicolas@gmail.com


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