MAINTAINERS: add an entry for the PHC code

Some months ago, tglx asked me off list if I would maintain the PTP
Hardware Clock code. Since then, the code has been fully merged, and I
am actively looking after it. This patch makes it official.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>