arm: initial Xen support

- Basic hypervisor.h and interface.h definitions.
- Skeleton enlighten.c, set xen_start_info to an empty struct.
- Make xen_initial_domain dependent on the SIF_PRIVILIGED_BIT.

The new code only compiles when CONFIG_XEN is set, that is going to be
added to arch/arm/Kconfig in patch #11 "xen/arm: introduce CONFIG_XEN on
ARM".

Changes in v3:

- improve comments.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>