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
diff --git a/spm/cactus/cactus_main.c b/spm/cactus/cactus_main.c
index 58186d1..dd15d97 100644
--- a/spm/cactus/cactus_main.c
+++ b/spm/cactus/cactus_main.c
@@ -206,6 +206,9 @@
 
 		/* Configure and enable Stage-1 MMU, enable D-Cache */
 		cactus_plat_configure_mmu(ffa_id);
+
+		/* Initialize locks for tail end interrupt handler */
+		sp_handler_spin_lock_init();
 	}
 
 	/*