soc: Add audio backend to hwdep interface.

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

CRs-Fixed: 661018
Change-Id: Id38cce4be2cebd89bbedc100553c733aa9e90739
Signed-off-by: Subhash Chandra Bose Naripeddy <snariped@codeaurora.org>
Signed-off-by: Krishnankutty Kolathappilly <kkolat@codeaurora.org>