ASoC: rsnd: add HDMI output support

Renesas R-Car Gen3 can output HDMI sound if SSIU/SSI are connected to
R-Car built-in HDMI device (R-Car Gen3 built-in HDMI device will be
controlled by DRM/KMS driver).
If SSIx was connected to HDMI0/1 on DT, SSI driver will detect it
automatically by this patch.
Note is that now Renesas R-Car sound driver is assuming that it is
using OF-graph base simple card for HDMI sound.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>