Use secure timer to generate S-EL1 interrupts ...

This patch adds support in the TSP to program the secure physical
generic timer to generate a EL-1 interrupt every half second. It also
adds support for maintaining the timer state across power management
operations. The TSPD ensures that S-EL1 can access the timer by
programming the SCR_EL3.ST bit.

This patch does not actually enable the timer. This will be done in a
subsequent patch once the complete framework for handling S-EL1
interrupts is in place.

Change-Id: I1b3985cfb50262f60824be3a51c6314ce90571bc