arm: introduce a DTS for Xen unprivileged virtual machines

arm: introduce a DTS for Xen unprivileged virtual machines

Given that the xenvm machine is based on vexpress but with an extremely
limited selection of peripherals (the guest is supposed to use virtual
devices instead), add "xen,xenvm" to the list of compatible machines in
mach-vexpress.

Changes in v3:

- add comments to mark fields that are likely to be changed by the
hypervisor.

Changes in v2:

- remove include skeleton;
- use #address-cells = <2> and #size-cells = <2>;
- remove the debug bootargs;
- use memory@80000000 instead of memory;
- remove the ranges and interrupt-map from the motherboard node;
- set the machine compatible to "xen,xenvm-4.2", "xen,xenvm";
- rename the dts file to xenvm-4.2.dts;
- add "xen,xenvm" to the list of compatible DT strings to mach-vexpress.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pawel Moll <pawel.moll@arm.com> (v2m changes)