ARM64: XEN: Add a function to initialize Xen specific UEFI runtime services

ARM64: XEN: Add a function to initialize Xen specific UEFI runtime services

When running on Xen hypervisor, runtime services are supported through
hypercall. Add a Xen specific function to initialize runtime services.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit be1aaf4e4026118e4191117a48f8a8078d1c0ed4)
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Orabug: 24820937