Add new API - audio_set_volume_ratio

Add new API - audio_set_volume_ratio

This new API is to set the individual volume ratio of
each stream. The parameters of this API consist of
stream role, direction, index and its ratio.

Because the stream role can differ among the streams
which have same volume type, this new API could be
used where the H/W volume has to be set for a specific
stream role with distinguishing it from others.

[Version] 0.1.1
[Issue Type] New API

Change-Id: Id2d35aaef4a6db489e8fb27cb6c4affe10b81dd4
Signed-off-by: Sangchul Lee <sc11.lee@samsung.com>