Non-Secure Interrupt support during Standard SMC processing in TSP

Implements support for Non Secure Interrupts preempting the
Standard SMC call in EL1. Whenever an IRQ is trapped in the
Secure world we securely handover to the Normal world
to process the interrupt. The normal world then issues
"resume" smc call to resume the previous interrupted SMC call.
Fixes ARM-software/tf-issues#105

Change-Id: I72b760617dee27438754cdfc9fe9bcf4cc024858
Merge pull request #83 from athoelke:at/tf-issues-126
Merge pull request #67 from achingupta:ag/psci_standby_bug_fix