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/xtensa/lib/time.c b/arch/xtensa/lib/time.c
index 81459b4..62bbe37 100644
--- a/arch/xtensa/lib/time.c
+++ b/arch/xtensa/lib/time.c
@@ -4,6 +4,7 @@
  */
 
 #include <common.h>
+#include <time.h>
 #include <asm/global_data.h>
 #include <linux/stringify.h>