ASoC: dapm: Add kcontrol support for PGAs

For DSPs we can define processing blocks as DAPM PGA widgets.
Some of these proceesing blocks can be configured by usermode
like EQ etc. So we need to add support of kcontrol for PGA
widgets.

Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Mythri P K <mythri.p.k@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>