mixerendpoint: Add pad templates for internal mixer pads

Change-Id: I0cfd71cf51f7e799a33e8bf9b1f11661ff1de376
mixerendpoint: Add basic test

Change-Id: I8d8864a5333b57b278e75ee6c9db7a3b2230e1ec
mixerendpoint: Manage mixer src pads depending on kmselement valves

Change-Id: I5cbd0eeb717bb842d93ec6b3fad9b8440ce40e56
mixerendpoint: Add test for mixer_src pads creation

Change-Id: I51d9bd56db52b94eec0fcf28b9494f7073afc11d