arm64: dts: rockchip: Add HDMI audio nodes to rk356x

This adds the i2s0 node and an hdmi-sound sound device to the
rk356x device tree. On the rk356[68], the i2s0 controller is
connected to HDMI audio.

Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Tested-by: Peter Geis <pgwipeout@gmail.com>
Signed-off-by: Nicolas Frattaroli <frattaroli.nicolas@gmail.com>
Link: https://lore.kernel.org/r/20220611065300.885212-2-frattaroli.nicolas@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>