diag: Add notification for DCI

Until now, DIAG user space clients were unaware of smd
channel state in kernel. This change will add a notification
mechanism for DCI clients. This will notify the client if any
DCI smd channel comes up.

Change-Id: I043ab15505b40a23ba0e6ac6488ddfa8d80ae4a7
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>