irqchip/gic: Drop support for secondary GIC in non-DT systems

We do not have any in-tree platform with this pathological setup,
and only a single system (Cavium's cns3xxx) isn't DT aware.

Let's drop the secondary GIC support for now, until we remove
the above horror altogether.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>