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.

However we had a few nasty bugs in the magic page implementation on vcpus
newer than 970 (p7, p8) that KVM now has workarounds for. It indicates that
it does have these workarounds through the PPC_FIXUP_HCALL capability.

To not expose broken guest kernels to issues on host kernels that don't
have the fixups in place, we don't expose working hypercall instructions
when the fixups are not available so that the guest can never active the
magic page.

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