When running on Xen hypervisor, runtime services are supported through
hypercall. Add a Xen specific function to initialize runtime services.
Signed-off-by: NShannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: NJulien Grall <julien.grall@arm.com>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>