ALSA: PCM: User control API implementation

Introduced a new helper function snd_pcm_add_usr_ctls() to
create control elements representing the user control for each
PCM (sub)stream

Change-Id: I9f69470ff2c5d6df8d1fdc2e1c2caf198076076e
Signed-off-by: Jayasena Sangaraboina <jsanga@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>
Signed-off-by: Meng Wang <mwang@codeaurora.org>