sunxi: HYP/non-sec: add sun7i PSCI backend

So far, only supporting the CPU_ON method.
Other functions can be added later.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>