powerpc/powernv: panic() on OPAL < V3

The OpenPower Abstraction Layer firmware went through a couple
of iterations in the lab before being released. What we now know
as OPAL advertises itself as OPALv3.

OPALv2 and OPALv1 never made it outside the lab, and the possibility
of anyone at all ever building a mainline kernel today and expecting
it to boot on such hardware is zero.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>