irqchip/gicv3-its: Sets IRQ_DOMAIN_FLAG_MSI_REMAP

The GICv3 ITS is MSI remapping capable. Let's advertise
this property so that VFIO passthrough can assess IRQ safety.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Tomasz Nowicki <tomasz.nowicki@caviumnetworks.com>
Tested-by: Tomasz Nowicki <tomasz.nowicki@caviumnetworks.com>
Tested-by: Bharat Bhushan <bharat.bhushan@nxp.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>