OMAPDSS: HDMI: Add op to get audio DMA port address offset

It could be possible that the DMA port differs accross diferent HDMI IPs. Thus,
add an IP-specific function to obtain the address offset and size of the DMA
data port.

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