xen/arm: gic-v2: Only create GICv2m node when there are GICv2m frame available

xen/arm: gic-v2: Only create GICv2m node when there are GICv2m frame available

Xen will crash on platform where GICv2m is not available with the
following error:

(XEN) Can't find ranges property for the gic node
(XEN) Device tree generation failed (-15).
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Could not set up DOM0 guest OS
(XEN) ****************************************

This is because the property "ranges" may not be present in the GIC
when there are no GICv2m frames.

Skip the creation of the GICv2m node when the hardware does not
support it.

This fixes boot after commit "xen/arm: Export GICv2m register frames to
DOM0 by device tree".

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Wei Chen <wei.chen@linaro.org>