powerpc/powernv: Remove OPALv2 firmware define and references

OPALv2 only ever existed in the lab and didn't escape to the world.
All OPAL systems in the wild are OPALv3.

The probability of there being an OPALv2 system still powered on
anywhere inside IBM is approximately zero, let alone anyone
expecting to run mainline kernels.

So, start to remove references to OPALv2.

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