misc: apq8084_dock: Add notifier chain support

Add an API to allow for interested clients to register
a notifier_block that gets called back when the tablet
is inserted and removed from the dock.

Change-Id: I30ccb78c10fda9db03b86c6882553063aedd68d4
Signed-off-by: Jack Pham <jackp@codeaurora.org>