powerpc/32s: Only build hash code when CONFIG_PPC_BOOK3S_604 is selected

It is now possible to only build book3s/32 kernel for
CPUs without hash table.

Opt out hash related code when CONFIG_PPC_BOOK3S_604 is not selected.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/62df436454ef06e104cc334a0859a2878d7888d5.1608274548.git.christophe.leroy@csgroup.eu