test(el3-runtime): dit is retained on world switch

Add tsp service to check the value of the PSTATE DIT bit is as
expected and toggle it's value. This is used to ensure that
the DIT bit is maintained during a switch from the Normal to
Secure worlds and back.

Change-Id: I4e8bdfa6530e5e75925c0079d4fa2795133c5105
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>