ASoC: Intel: Headset button support in broxton machine driver

Map the 4 headset buttons to KEY_PAUSE, KEY_VOLUMEUP, KEY_VOLUMEDOWN and
KEY_VOICECOMMAND.

Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Naveen Manohar <naveen.m@intel.com>
Signed-off-by: Yong Zhi <yong.zhi@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>