powerpc/pseries: Add a warning in the case of cross-cpu VPA registration

The spec says it "may be problematic" if CPU x registers the VPA of
CPU y. Add a warning in case we ever do that.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>