drm: bridge/dw_hdmi-ahb-audio: basic support for multi-channel PCM audio

Add basic support for multi-channel PCM audio, with fixed speaker
mappings.  This has been tested with an AV receiver, and appears to
work for low sample rates up to 8 channels.

It should be noted that multi-channel mode using the IEC958 alsa-lib
conversion plugin requires correct AES channel status for the AV
receiver to recognise the stream, especially the sample rate bits.
"Not identified" does not work there.

Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>