Pass 'cookie' parameter to interrupt handler in BL3-1

The interrupt handling routine in BL3-1 expects a cookie as its last
parameter which was not being passed when invoking the interrupt
handler in BL3-1. This patch fixes that by passing a dummy cookie
parameter in the x3 register.

Fixes ARM-software/tf-issues#171

Change-Id: Ic98abbbd9f849e6f1c55343e865b5e0a4904a1c5

bl31/aarch64/runtime_exceptions.S

@@ -125,6 +125,9 @@
 	/* Restore the reference to the 'handle' i.e. SP_EL3 */
 	mov	x2, x20
 
+	/*  x3 will point to a cookie (not used now) */
+	mov	x3, xzr
+
 	/* Call the interrupt type handler */
 	blr	x21