snd-bcm2835: Add support for spdif/hdmi passthrough

This adds a dedicated subdevice which can be used for passthrough of non-audio
formats (ie encoded a52) through the hdmi audio link. In addition to this
driver extension an appropriate card config is required to make alsa-lib
support the AES parameters for this device.
rpi: Add ProTUNE default configuration.

Signed-off-by: Julian Scheel <julian@jusst.de>