powerpc/mm: Move slb_addr_linit to early_init_mmu

Avoid #ifdef in generic code. Also enables us to do this specific to
MMU translation mode on book3s64

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>