powerpc/boot: don't clobber r6 and r7 in epapr boot

We use r6 and r7 for epapr boot, but the current pre-C init will clobber
both of these.

This change does a simple replacement, of r6 -> r12 and r7 -> r13, so
that we hit platform init with these registers intact.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>