msm: smd: add smd_edge_to_local_pid() private api

Add a private API function called smd_edge_to_local_pid() to allow SMD
helper drivers to get the processor id assigned to the local half of a
specified SMD edge.

Change-Id: Ibfad02b3af6a9dff87558797274f45f516d96428
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>