ASoC: rockchip-spdif: add #sound-dai-cells property

'#sound-dai-cells' is required to properly interpret
the list of DAI specified in the 'sound-dai' property,
so add them to 'rockchip-spdif.yaml'

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200404115225.4314-2-jbx6244@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>