msm: camera: reqmgr: Add uapi for new v4l2 event type

Add event type to be notify custom events from the custom
driver. Also adds a new custom message definition.

CRs-Fixed: 2569823
Change-Id: I2ff701e79949ac3a467cbfe0c704065dbf0dc759
Signed-off-by: Karthik Anantha Ram <kartanan@codeaurora.org>