powerpc: Ensure that ppc_md is empty before probing for machine type

Anything in there will be overwritten, so it helps catching nasty
bugs if we check that it's indeed full of NULL's before we do so.

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