soc: Add audio backend to hwdep interface

Add audio backend to hwdep interface. This is to provide ioctl
interface to communicate to routing driver.

Change-Id: I9abeb9cdf3ea58d9fac13eef5d849c8aadf1bfbf
Signed-off-by: Subhash Chandra Bose Naripeddy <snariped@codeaurora.org>
Signed-off-by: Krishnankutty Kolathappilly <kkolat@codeaurora.org>
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Sudheer Papothi <spapothi@codeaurora.org>