arm,arm64/xen: introduce HYPERVISOR_suspend()

Introduce HYPERVISOR_suspend() and a few additional empty stubs for
Xen arch specific functions called by drivers/xen/manage.c.

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