dt-bindings: display: sii902x: Add HDMI audio bindings

The sii902x chip family supports also HDMI audio. Add binding for
describing the necessary i2s and mclk wiring for it.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f790277fabc45f987f90075dd842368705226a84.1558964241.git.jsarha@ti.com