timer: Allow callers to pass 'now' to timestamp_expired()

To avoid calling get_time() when the caller already knows the value, add
a parameter to timestamp_expired().

BUG=chrome-os-partner:9424
TEST=build and boot on Daisy

Change-Id: Ibb97c86f429ec4b814e17b41cbf79b612a75097a
Signed-off-by: Simon Glass <sjg@chromium.org>