arm: Move tbu to arch_global_data

Move this field into arch_global_data and tidy up.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/cpu/armv7/u8500/timer.c b/arch/arm/cpu/armv7/u8500/timer.c
index 79aad99..bb9165b 100644
--- a/arch/arm/cpu/armv7/u8500/timer.c
+++ b/arch/arm/cpu/armv7/u8500/timer.c
@@ -132,7 +132,7 @@
 /*
  * Emulation of Power architecture long long timebase.
  *
- * TODO: Support gd->tbu for real long long timebase.
+ * TODO: Support gd->arch.tbu for real long long timebase.
  */
 unsigned long long get_ticks(void)
 {