powerpc/mm: Remove the dependency on pte bit position in asm code

We should not expect pte bit position in asm code. Simply
by moving part of that to C

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