PPC: spapr: Expose /hypervisor node in device tree

PR KVM supports an ePAPR compliant hypercall interface in parallel to the
normal sPAPR one. Expose the ePAPR /hypervisor node and properties to the
guest so it can use it.

This enables magic page sharing on PR KVM with -M pseries.

Signed-off-by: Alexander Graf <agraf@suse.de>