[POWERPC] Make the debugfs "powerpc" dir globally accessible

The prom.c debugging code creates a "powerpc" directory in debugfs,
which is nice, but doesn't allow any other debugging code to stick things
under "powerpc" in debugfs. So make it global.

While we're there we should make the prom.c debugging code depend on
CONFIG_DEBUG_FS, because it doesn't work otherwise.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/include/asm-powerpc/system.h b/include/asm-powerpc/system.h
index eff3de9..32aa42b 100644
--- a/include/asm-powerpc/system.h
+++ b/include/asm-powerpc/system.h
@@ -559,5 +559,7 @@
 extern void account_system_vtime(struct task_struct *);
 #endif
 
+extern struct dentry *powerpc_debugfs_root;
+
 #endif /* __KERNEL__ */
 #endif /* _ASM_POWERPC_SYSTEM_H */