OMAPDSS: HDMI: Create platform device for audio support

Creating the accessory devices, such as audio, from the HDMI driver
allows to regard HDMI as a single entity with audio an display
functionality. This intends to follow the design of drivers such
as MFD, in which a single entity handles the creation of the accessory
devices. Such devices are then used by domain-specific drivers; audio in
this case.

Also, this is in line with the DT implementation of HDMI, in which we will
have a single node to describe this feature of the OMAP SoC.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>