[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>
diff --git a/include/asm-powerpc/machdep.h b/include/asm-powerpc/machdep.h
index 71c6e7e..cc7c17f 100644
--- a/include/asm-powerpc/machdep.h
+++ b/include/asm-powerpc/machdep.h
@@ -99,7 +99,7 @@
 #endif /* CONFIG_PPC64 */
 
 	int		(*probe)(void);
-	void		(*setup_arch)(void);
+	void		(*setup_arch)(void); /* Optional, may be NULL */
 	void		(*init_early)(void);
 	/* Optional, may be NULL. */
 	void		(*show_cpuinfo)(struct seq_file *m);