powerpc/mm: PTE_ATOMIC_UPDATES is only for 40x

Only 40x still uses PTE_ATOMIC_UPDATES.
40x cannot not select CONFIG_PTE64_BIT.

Drop handling of PTE_ATOMIC_UPDATES:
- In nohash/64
- In nohash/32 for CONFIG_PTE_64BIT

Keep PTE_ATOMIC_UPDATES only for nohash/32 for !CONFIG_PTE_64BIT

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d6f8e1f46583f1842de24581a68b0496feb15516.1589866984.git.christophe.leroy@csgroup.eu