OMAPDSS: hdmi.h: Add HDMI_AUDIO_LAYOUT_6CH enum value

The OMAP5 HDMI audio implementation needs HDMI_AUDIO_LAYOUT_6CH in
hdmi_core_audio_layout enum. I found the correct value from ti-linux
3.8 tree.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>