ASoC: dt-bindings: rockchip: convert rk3399-gru-sound to DT Schema

ASoC: dt-bindings: rockchip: convert rk3399-gru-sound to DT Schema

Convert the rockchip,rk3399-gru-sound.txt DT binding to DT Schema
format.

Update rockchip,cpu from a single I2S controller phandle to a
phandle-array. Add an optional second entry for the SPDIF controller,
as seen in rk3399-gru.dtsi, required by boards with DisplayPort audio.

Signed-off-by: Anushka Badhe <anushkabadhe@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260410055532.60868-1-anushkabadhe@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>