MAINTAINERS: Update timer related entries

Bring the existing file list up to date and add a new entry for
timekeeping and ntp. Assign John Stultz to this new entry so he gets
all the blame :)

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <johnstul@us.ibm.com>