msm: msm_bus: Add support for registering multiple clients

Currently, the bus driver assumes only one client can be
registered using a single device-tree node.
Fix this limitation by adding capability to add multiple
clients using a single device-tree node associated with
the platform device.

Change-Id: I3de972fc81515d4f8571409d6593b0a3645789db
Signed-off-by: Gagan Mac <gmac@codeaurora.org>