powerpc/64: Move 64-bit probe_machine() to later in the boot process

We no long need the machine type that early, so we can move probe_machine()
to after the device-tree has been expanded. This will allow further
consolidation.

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