powerpc: Add second POWER8 PVR entry

POWER8 comes with two different PVRs.  This patch enables the additional
PVR in the cputable.

The existing entry (PVR=0x4b) is renamed to POWER8E and the new entry
(PVR=0x4d) is given POWER8.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>