common: Move get_ticks() function out of common.h

This function belongs in time.h so move it over and add a comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/cpu/armv7/stv0991/timer.c b/arch/arm/cpu/armv7/stv0991/timer.c
index d1b763d..695bdd7 100644
--- a/arch/arm/cpu/armv7/stv0991/timer.c
+++ b/arch/arm/cpu/armv7/stv0991/timer.c
@@ -5,6 +5,7 @@
  */
 
 #include <common.h>
+#include <time.h>
 #include <asm/io.h>
 #include <asm/arch-stv0991/hardware.h>
 #include <asm/arch-stv0991/stv0991_cgu.h>