powerpc: Drop support for pre-POWER4 cpus

We inadvertently broke power3 support back in 3.4 with commit
f5339277 "powerpc: Remove FW_FEATURE ISERIES from arch code".
No one noticed until at least 3.9.

By then we'd also broken it with the optimised memcpy, copy_to/from_user
and clear_user routines. We don't want to add any more complexity to
those just to support ancient cpus, so it seems like it's a good time to
drop support for power3 and earlier.

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