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-zynq/timer.c b/arch/arm/mach-zynq/timer.c
index 8658abb..211ea15 100644
--- a/arch/arm/mach-zynq/timer.c
+++ b/arch/arm/mach-zynq/timer.c
@@ -31,6 +31,7 @@
 #include <common.h>
 #include <div64.h>
 #include <dm.h>
+#include <time.h>
 #include <asm/io.h>
 #include <asm/arch/hardware.h>
 #include <asm/arch/clk.h>