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