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>

Documentation/devicetree/bindings/arm/gic-v3.txt

@@ -57,6 +57,8 @@ used to route Message Signalled Interrupts (MSI) to the CPUs.
 These nodes must have the following properties:
 - compatible : Should at least contain  "arm,gic-v3-its".
 - msi-controller : Boolean property. Identifies the node as an MSI controller
+- #msi-cells: Must be <1>. The single msi-cell is the DeviceID of the device
+  which will generate the MSI.
 - reg: Specifies the base physical address and size of the ITS
   registers.
 
@@ -83,6 +85,7 @@ Examples:
 		gic-its@2c200000 {
 			compatible = "arm,gic-v3-its";
 			msi-controller;
+			#msi-cells = <1>;
 			reg = <0x0 0x2c200000 0 0x200000>;
 		};
 	};
@@ -107,12 +110,14 @@ Examples:
 		gic-its@2c200000 {
 			compatible = "arm,gic-v3-its";
 			msi-controller;
+			#msi-cells = <1>;
 			reg = <0x0 0x2c200000 0 0x200000>;
 		};
 
 		gic-its@2c400000 {
 			compatible = "arm,gic-v3-its";
 			msi-controller;
+			#msi-cells = <1>;
 			reg = <0x0 0x2c400000 0 0x200000>;
 		};
 	};