powerpc/mm/radix: Use different RTS encoding for different POWER9 revs

POWER9 DD1 uses RTS - 28 for the RTS value but other revisions use
RTS - 31.  This makes this distinction for the different revisions

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