irqchip/gic-v3-its: Tag ITS device as shared if allocating for a proxy device

The ITS already has some notion of "shared" devices. Let's map the
MSI_ALLOC_FLAGS_PROXY_DEVICE flag onto this internal property.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/20201129135208.680293-3-maz@kernel.org