[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/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index 3089eae..008ab68 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -530,7 +530,8 @@
 	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");