powerpc: Add asm/debug.h to get powerpc_debugfs_root

Since the "Disintegrate asm/system.h for PowerPC"
(ae3a197e) This has been failing when
DEBUG is #defined.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>