xen/arm: Xen detection and shared_info page mapping

Check for a node in the device tree compatible with "xen,xen", if it is
present set xen_domain_type to XEN_HVM_DOMAIN and continue
initialization.

Map the real shared info page using XENMEM_add_to_physmap with
XENMAPSPACE_shared_info.

Changes in v4:

- simpler parsing of Xen version in the compatible DT node.

Changes in v3:

- use the "xen,xen" notation rather than "arm,xen";
- add an additional check on the presence of the Xen version.

Changes in v2:

- replace pr_info with pr_debug.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>