gicv2: Fix support for systems without secure interrupts

Accessing the interrupt_props array only happens inside a loop over
interrupt_props_num, so the GICv2 driver can cope with no secure
interrupts. As in fact we have already some asserts in place that
respect that, lets change the final place where we insist on a non-NULL
pointer to relax that.
This enables GICv2 platforms which have no need for a secure interrupt.

This only covers the non-deprecated code paths.

Also we remove a now redundant assert().

Change-Id: Id100ea978643d8558335ad28649d55743fe9bd4c
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>