msm: synx: provide function to enable external signaling

Provide a function to allow the external clients to enable
and/or set-up the external synchronization on the object
before signaling.
This function is optional, and is invoked before calling the
external signal functionality.

Set up enable signal functionality for the camera sync driver.

Change-Id: Ib01adebeae7bd82b59ba62d168d966449fcc5e6f
Signed-off-by: Sumukh Hallymysore Ravindra <shallymy@codeaurora.org>