time: move timer APIs to include/time.h

The include/common.h is a collection of unrelated declarations,
macros, etc.

It is horrible to include such a cluttered header just for some
timer functions.  Split out timer functions into include/time.h.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>