MAINTAINERS: Add Stephen Boyd as timekeeping reviewer

After showing expertise and presenting on the timekeeping
subsystem at ELC[1], Stephen clearly should be included in
the maintainer list.

[1] https://www.youtube.com/watch?v=Puv4mW55bF8



Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>