xen: arm: create dom0 DTB /hypervisor/ node dynamically.

xen: arm: create dom0 DTB /hypervisor/ node dynamically.

I initially added hypervisor-new and confirmed via /proc/device-model
that the content is the same before changing it to drop and replace
an existing node.

NB: There is an ambiguity in the compatibility property.
linux/arch/arm/boot/dts/xenvm-4.2.dts says "xen,xen-4.2" while
Documentation/devicetree/bindings/arm/xen.txt says "xen,xen-4.3". I have
used the actual hypervisor version as discussed in
http://marc.info/?l=xen-devel&m=135963416631423

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>