doc: Document timer API

This adds kerneldocs for <timer.h>.

I don't know who should maintain doc/api/timer.rst, since the timer
subsystem seems to be maintained by SoC maintainers. MAINTAINERS is left
un-updated for the moment.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
3 files changed