MAINTAINERS: Add entry for kernel/time/alarmtimer.c

I've been missing patches against alarmtimer.c due to
a lack of a proper entry for it in the MAINTAINERS file.

So update MAINTAINERS to fix this, adding it in with the
timekeeping, ntp and core clocksource logic I share with
Thomas.

Signed-off-by: John Stultz <john.stultz@linaro.org>