ABORT SMC used to return to the previously executing world, which
happened to be S-EL1 as it calls a TSP handler using synchronous entry
into the TSP.

Now properly save and restore the non-secure context (including system
registers) and return to non-secure world as it should.

fixes ARM-Software/tf-issues#453

Change-Id: Ie40c79ca2636ab8b6b2ab3106e8f49e0f9117f5f
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>