arm: use static DT phandle for the GIC

As KVM supports only onc (v)GIC per guest and it's hard to imagine that
we will ever need more than that, lets simplify the FDT generation by
not passing that single, constant phandle around.
Let's just reference that one global symbol from enum phandles instead.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>