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>

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 30eae87..f42968a 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -251,6 +251,7 @@
 core-$(CONFIG_FPE_NWFPE)	+= arch/arm/nwfpe/
 core-$(CONFIG_FPE_FASTFPE)	+= $(FASTFPE_OBJ)
 core-$(CONFIG_VFP)		+= arch/arm/vfp/
+core-$(CONFIG_XEN)		+= arch/arm/xen/
 
 # If we have a machine-specific directory, then include it in the build.
 core-y				+= arch/arm/kernel/ arch/arm/mm/ arch/arm/common/