ALSA: PCM: volume API implementation

Introduced a new helper function snd_pcm_add_volume_ctls() to
create control elements representing the volume for each PCM
(sub)stream.

Change-Id: Id483257c4fe84a909e7a4af3a5aa3e4f0d5d9ea5
Signed-off-by: Damir Didjusto <damird@codeaurora.org>