powerpc/pgtable: Drop PTE_ATOMIC_UPDATES

40x was the last user of PTE_ATOMIC_UPDATES.

Drop everything related to PTE_ATOMIC_UPDATES.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/dbe8438fd1ed3e500132c8ab70269d4e6cc84531.1590079968.git.christophe.leroy@csgroup.eu