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>

include/xen/xen.h

@@ -23,7 +23,7 @@ extern enum xen_domain_type xen_domain_type;
 #include <xen/interface/xen.h>
 #include <asm/xen/hypervisor.h>
 
-#define xen_initial_domain()	(xen_pv_domain() && \
+#define xen_initial_domain()	(xen_domain() && \
 				 xen_start_info->flags & SIF_INITDOMAIN)
 #else  /* !CONFIG_XEN_DOM0 */
 #define xen_initial_domain()	(0)