[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>

arch/powerpc/kernel/setup_32.c

@@ -290,7 +290,8 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;
 #endif
 
-	ppc_md.setup_arch();
+	if (ppc_md.setup_arch)
+		ppc_md.setup_arch();
 	if ( ppc_md.progress ) ppc_md.progress("arch: exit", 0x3eab);
 
 	paging_init();

arch/powerpc/kernel/setup_64.c

@@ -530,7 +530,8 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;
 #endif
 
-	ppc_md.setup_arch();
+	if (ppc_md.setup_arch)
+		ppc_md.setup_arch();
 
 	paging_init();
 	ppc64_boot_msg(0x15, "Setup Done");

include/asm-powerpc/machdep.h

@@ -99,7 +99,7 @@ struct machdep_calls {
 #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);