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>

drivers/mailbox/mailbox-uclass.c

@@ -7,6 +7,7 @@
 #include <dm.h>
 #include <mailbox.h>
 #include <mailbox-uclass.h>
+#include <time.h>
 
 static inline struct mbox_ops *mbox_dev_ops(struct udevice *dev)
 {