msm: msm_bus: Add new APIs for bus scaling

Add new APIs to the bus scaling driver. The new APIs make it
easier for clients to setup paths for bus scaling. The driver APIs
will return a pointer to a client handle in case of success and NULL or
error in cases of failure. For now the existing APIs will remain as is
eventually all clients will start switching over to the new APIs.

Change-Id: I22656dddf13802128ee5c4faab9f83f9c6f8e683
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>