powerpc/mm: Move common pte bits and accessors to book3s/64/pgtable.h

Now that we have moved book3s hash64 Linux pte bits to match Power ISA
3.0 radix pte bit positions, we move the matching pte bits to a common
header.

Only code movement in this patch. No functionality change.

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