powerpc: Remove old Makefile workaround for arch/ppc

There is an old workaround in the sysdev/Makefile for dealing
with arch/ppc vs. arch/powerpc compiles.  This is no longer
needed as arch/ppc is dead.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>