powerpc/mm: Clear top 16 bits of va only on older cpus

As per ISA, we need to do this only for architecture version 2.02 and
earlier. This continued to work even for 2.07. But let's not do this for
anything after 2.02. ISA 3.0 requires these top bits to be not cleared.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>