ASoC: Intel: Skylake: Add multichannel support for HDMI

Channel max is changed to 8 from stereo to support multichannel
capability for HDMI devices.

Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>