MAINTAINERS: add keyword match on PTP

Most of PTP drivers live under ethernet and we have to keep
telling people to CC the PTP maintainers. Let's try a keyword
match, we can refine as we go if it causes false positives.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>