xenvm: add a simple PSCI node and a second cpu

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
CC: rob.herring@calxeda.com
CC: will.deacon@arm.com
CC: arnd@arndb.de