gicv3: Fix support for systems without secure interrupts

Accessing the interrupt_props array only happens inside a loop over
interrupt_props_num, so the GICv3 driver can cope with no secure
interrupts. This allows us to relax the asserts that insists on
a non-NULL interrupt_props pointer and at least one secure interrupt.
This enables GICv3 platforms which have no need for a secure interrupt.

This only covers the non-deprecated code paths.

Change-Id: I49db291906512f56af065772f69acb281dfbdcfb
Signed-off-by: Andre Przywara <andre.przywara@arm.com>