ASoC: hdac_hdmi: Add MST verb support

To support DP MST audio, new pin verbs/params are added. This patch
adds helper functions to do following:
o To set a specific port
o To get the currently selected port
o To get the length of port.

Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>