fwk: SCP suspend execution until an interrupt occurs

Currently, the firmware is busy looping forever while
waiting for any interrupts. This behaviour consumes power
unnecessarily. To avoid the busy loop scenario the firmware
will enter suspend state using WFE (Wait For Event)
instruction. Any interrupt will wake up the CPU to handle
new events.

Signed-off-by: Tarek El-Sherbiny <tarek.el-sherbiny@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>
Change-Id: I83508dcce721a6d3fb93fa598d03a4bb01331f00