feat(gic): add APIs to raise NS and S-EL1 SGIs

This patch adds two helper functions:
 - plat_ic_raise_ns_sgi to raise a NS SGI
 - plat_ic_raise_s_el1_sgi to raise a S-EL1 SGI

Signed-off-by: Florian Lugou <florian.lugou@provenrun.com>
Change-Id: I6f262dd1da1d77fec3f850eb74189e726b8e24da