microblaze_mmu_v2: Add CURRENT_TASK for entry.S

Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/include/asm/current.h

 /*
+ * Copyright (C) 2008-2009 Michal Simek <monstr@monstr.eu>
+ * Copyright (C) 2008-2009 PetaLogix
  * Copyright (C) 2006 Atmark Techno, Inc.
  *
  * This file is subject to the terms and conditions of the GNU General Public
@@ -9,6 +11,12 @@
 #ifndef _ASM_MICROBLAZE_CURRENT_H
 #define _ASM_MICROBLAZE_CURRENT_H
 
+/*
+ * Register used to hold the current task pointer while in the kernel.
+ * Any `call clobbered' register without a special meaning should be OK,
+ * but check asm/microblaze/kernel/entry.S to be sure.
+ */
+#define CURRENT_TASK	r31
 # ifndef __ASSEMBLY__
 /*
  * Dedicate r31 to keeping the current task pointer