docs: Xen ARM DT bindings

Add a doc to describe the Xen ARM device tree bindings


Changes in v5:

- add a comment about the size of the grant table memory region;
- add a comment about the required presence of a GIC node;
- specify that the described properties are part of a top-level
"hypervisor" node;
- specify #address-cells and #size-cells for the example.


Changes in v4:

- "xen,xen" should be last as it is less specific;
- update reg property using 2 address-cells and 2 size-cells.


Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
CC: devicetree-discuss@lists.ozlabs.org
CC: David Vrabel <david.vrabel@citrix.com>
CC: Rob Herring <robherring2@gmail.com>
CC: Dave Martin <dave.martin@linaro.org>