Docs: dt: add #msi-cells to GICv3 ITS binding

The GICv3 ITS uses sideband master identification data (known as a
DeviceID) to identify which master wrote to a doorbell, and this data is
used to determine how to react in response to the write.

Commit 1e6db000 ("irqchip/gicv3-its: Add platform MSI support")
added support per this binding, but failed to update the documentation.
This patch fixes the documentation.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>