powerpc/Makefile: Don't use $(ARCH) unnecessarily

There's no need to use $(ARCH) for references to the arch directory in
the source tree, it is always arch/powerpc.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231206115548.1466874-1-mpe@ellerman.id.au