Move bootstage timer out of lib/time.c

The standalone example does not have get_timer() defined, so we cannot
rely on it being available.

Move the timer function into boootstage.c to avoid this problem.

This corrects a build breakage for the standalone example on some boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Matthias Fuchs <matthias.fuchs@esd.eu>