feat(spmd): introduce platform handler for Group0 interrupt

feat(spmd): introduce platform handler for Group0 interrupt

This patch introduces a handler for FVP platform to triage Group0
secure interrupts. Currently, it is empty but serves as a
placeholder for future Group0 interrupt sources.

Moreover, this patch also provides a dummy implementation of the
above mentioned platform hook for QEMU, corstone100, n1sdp and
hikey960 ports.

Change-Id: I01d3451408f47ac313b0af74046cce89f89b85bb
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>