ARM: 6496/1: GIC: Do not try to register more then NR_IRQS interrupts

This change limits number of GIC-originating interrupts to the
platform maximum (defined by NR_IRQS) while still initialising
all distributor registers.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>