powerpc/mm: ppc 603 doesn't need update_mmu_cache()

On powerpc 603, there is no hash table so get out of
update_mmu_cache() early.

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/6133e0f115d955fac4061536dab0fa7480a1c433.1565933217.git.christophe.leroy@c-s.fr