Add events that trigger before entry to normal/secure world.  The
events trigger after the normal/secure context has been restored.

Similarly add events that trigger after leaving normal/secure world.
The events trigger after the normal/secure context has been saved.

Change-Id: I1b48a7ea005d56b1f25e2b5313d77e67d2f02bc5
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>