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=Puv4mW55bF8Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>

MAINTAINERS

@@ -11099,6 +11099,7 @@ F:	drivers/power/supply/bq27xxx_battery_i2c.c
 TIMEKEEPING, CLOCKSOURCE CORE, NTP, ALARMTIMER
 M:	John Stultz <john.stultz@linaro.org>
 M:	Thomas Gleixner <tglx@linutronix.de>
+R:	Stephen Boyd <sboyd@codeaurora.org>
 L:	linux-kernel@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers/core
 S:	Supported