extcon: Add usage of blocking notifier chain

This change adds required APIs to register blocking notifier call
for blocking functionality using extcon framework.

Change-Id: I0e8e3b29d169ca833f85e42d2df98dd8380cbf46
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>