powerpc/64: Move MMU backend selection out of platform code

We move it into early_mmu_init() based on firmware features. For PS3,
we have to move the setting of these into early_init_devtree().

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>