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/arm/mach-zynqmp/cpu.c b/arch/arm/mach-zynqmp/cpu.c
index b90d08b..442427b 100644
--- a/arch/arm/mach-zynqmp/cpu.c
+++ b/arch/arm/mach-zynqmp/cpu.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <time.h>
 #include <asm/arch/hardware.h>
 #include <asm/arch/sys_proto.h>
 #include <asm/armv8/mmu.h>