microblaze: Use MICROBLAZE_TLB_SIZE in asm code

TLB size was hardcoded in asm code. This patch brings ability
to change TLB size only in one place. (mmu.h).

Signed-off-by: Michal Simek <monstr@monstr.eu>
diff --git a/arch/microblaze/kernel/head.S b/arch/microblaze/kernel/head.S
index e397d5d..ba2c033 100644
--- a/arch/microblaze/kernel/head.S
+++ b/arch/microblaze/kernel/head.S
@@ -134,7 +134,7 @@
 	 * virtual to physical.
 	 */
 	nop
-	addik	r3, r0, 63		/* Invalidate all TLB entries */
+	addik	r3, r0, MICROBLAZE_TLB_SIZE -1	/* Invalidate all TLB entries */
 _invalidate:
 	mts	rtlbx, r3
 	mts	rtlbhi, r0			/* flush: ensure V is clear   */