feat(interrupts): support for registering custom handler

This patch provides support for registering and unregistering
handler that is invoked by SP at the tail end of the virtual interrupt
processing.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: Ia12686361063bb680ff32b4f4bf90e0af2521c36
4 files changed