Just like for GICv4.0, each VPE has its own doorbell interrupt, and
thus an irqchip that manages them. Since the doorbell management is
quite different on GICv4.1, let's introduce an almost empty irqchip
the will get populated over the next new patches.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20191224111055.11836-10-maz@kernel.org