powerpc: Add asm/debug.h to get powerpc_debugfs_root

Since the "Disintegrate asm/system.h for PowerPC"
(ae3a197e3d0bfe3f4bf1693723e82dc018c096f3) 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>

arch/powerpc/kernel/prom.c

@@ -54,6 +54,7 @@
 #include <asm/kexec.h>
 #include <asm/opal.h>
 #include <asm/fadump.h>
+#include <asm/debug.h>
 
 #include <mm/mmu_decl.h>