This patch adds support in the TSP for handling S-EL1 interrupts
handed over by the TSPD. It includes GIC support in its platform port,
updates various statistics related to FIQ handling, exports an entry
point that the TSPD can use to hand over interrupts and defines the
handover protocol w.r.t what context is the TSP expected to preserve
and the state in which the entry point is invoked by the TSPD.

Change-Id: I93b22e5a8133400e4da366f5fc862f871038df39