powerpc/mm: Introduce early_init_mmu() on 64-bit

This moves some MMU related init code out of setup_64.c into hash_utils_64.c
and calls it early_init_mmu() and early_init_mmu_secondary(). This will
make it easier to plug in a new MMU type.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>