arm64: Juno: Add GICv2m support in device tree.

Juno contains a GICv2m extension for handling PCIe MSI messages.
Add a node declaring the first frame of the extension.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Jon Medhurst <tixy@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>