irqchip/gic-v3-its: Flag device allocation as proxied if behind a PCI bridge

An aliasing PCI bridge is another case where we should flag the
corresponding allocation as "proxied", as MSIs are coming with
the bridge's RID, and not the originating device's.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20201129135208.680293-4-maz@kernel.org