[POWERPC] Only call ppc_md.setup_arch() if it is provided

This allows platforms which don't have anything to do at setup_arch time
(like a bunch of the 4xx platforms) to eliminate an empty setup_arch hook.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>