powerpc/32s: Move early_mmu_init() into mmu.c

early_mmu_init() is independent of MMU type and not
directly linked to tlb handling.

In a following patch, tlb.c will be restricted to HASH mmu.

Move early_mmu_init() into mmu.c which is common.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e51b5e2fe6bca623b33116403043d3a1b5eaf826.1603348103.git.christophe.leroy@csgroup.eu