xen/arm: get privilege status

Use Xen features to figure out if we are privileged.

XENFEAT_dom0 was introduced by 23735 in xen-unstable.hg.

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