common: Move some time functions out of common.h

These functions belong in time.h so move them over and add comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/common.h

@@ -288,10 +288,6 @@ int cleanup_before_linux_select(int flags);
 uint64_t get_ticks(void);
 void	wait_ticks    (unsigned long);
 
-/* arch/$(ARCH)/lib/time.c */
-ulong	usec2ticks    (unsigned long usec);
-ulong	ticks2usec    (unsigned long ticks);
-
 /* lib/uuid.c */
 #include <uuid.h>