kmsaudiomixer: Add source pads

Change-Id: Id551bde722f3690f68e9c511ff91974486bbcae0
audiomixer: Test adition of input audio

Change-Id: I7b9b14cf73b9c5a79ea490f66881a2332ad64412
kmsaudiomixer: Unlink pads

Change-Id: I3b3c7e576dc53440895a77b3a0a909491b4004a7
kmsaudiomixer: Unlink adder elements when any input source is disconnected

Change-Id: I0f0ebe98997f8726aaa209532d570938a2d90459