ASoC: convert rockchip spdif bindings to yaml

Current dts files with 'spdif' nodes are manually verified.
In order to automate this process rockchip-spdif.txt
has to be converted to yaml.

Also rk3188.dtsi, rk3288.dtsi use an extra fallback string,
so change this in the documentation.

Changed:
"rockchip,rk3188-spdif", "rockchip,rk3066-spdif"
"rockchip,rk3288-spdif", "rockchip,rk3066-spdif"

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200404115225.4314-1-jbx6244@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>