common: Move get_tbclk() to time.h

This function related to timer and most of the timer functions are in
time.h, so move this function there.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/mips/cpu/time.c b/arch/mips/cpu/time.c
index af324f7..a1508e3 100644
--- a/arch/mips/cpu/time.c
+++ b/arch/mips/cpu/time.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <time.h>
 #include <asm/mipsregs.h>
 
 unsigned long notrace timer_read_counter(void)