SPM: Add interrupt handling support for SFN backend

This patch adds the interrupt handling support for SFN backend:
- Add two new ops in backend
- Add IRQ APIs for SFN interface

Change-Id: If5c694800f6c882f5852c9092143f4db5f0f4bd1
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
8 files changed