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/drivers/timer/tsc_timer.c b/drivers/timer/tsc_timer.c
index 637c8ff..0df551f 100644
--- a/drivers/timer/tsc_timer.c
+++ b/drivers/timer/tsc_timer.c
@@ -9,6 +9,7 @@
 #include <common.h>
 #include <dm.h>
 #include <malloc.h>
+#include <time.h>
 #include <timer.h>
 #include <asm/cpu.h>
 #include <asm/io.h>