powerpc: Create asm/debugfs.h and move powerpc_debugfs_root there

powerpc_debugfs_root is the dentry representing the root of the
"powerpc" directory tree in debugfs.

Currently it sits in asm/debug.h, a long with some other things that
have "debug" in the name, but are otherwise unrelated.

Pull it out into a separate header, which also includes linux/debugfs.h,
and convert all the users to include debugfs.h instead of debug.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/debug.h

@@ -8,8 +8,6 @@
 
 struct pt_regs;
 
-extern struct dentry *powerpc_debugfs_root;
-
 #if defined(CONFIG_DEBUGGER) || defined(CONFIG_KEXEC_CORE)
 
 extern int (*__debugger)(struct pt_regs *regs);