xen/docs: arm: Update dom0less binding and example

xen/docs: arm: Update dom0less binding and example

The binding for the dom0less module does not match Xen implementation.
Any module should contain the compatible "multiboot,module" to be
recognized.

This was clearly an oversight as other examples with Xen code base
provide the compatible "multiboot,module".

So fix the binding and the example associated to it.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>