xen/arm: introduce CONFIG_XEN on ARM

Changes in v5:

- make XEN_DOM0 depend on XEN;
- avoid "select XEN_DOM0" in XEN.


Changes in v2:

- mark Xen guest support on ARM as EXPERIMENTAL.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Sergei Shtylyov <sshtylyov@mvista.com>