target-ppc: Use sensible POWER8/POWER8E versions

We never released anything older than POWER8 DD2.0 and POWER8E DD2.1,
so let's use these versions, without that some firmware or Linux code
might fail to use some HW features that were non functional in earlier
internal only spins of the chip.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
