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>

MAINTAINERS

@@ -16669,6 +16669,7 @@ F:	Documentation/driver-api/ptp.rst
 F:	drivers/net/phy/dp83640*
 F:	drivers/ptp/*
 F:	include/linux/ptp_cl*
+K:	(?:\b|_)ptp(?:\b|_)
 
 PTP VIRTUAL CLOCK SUPPORT
 M:	Yangbo Lu <yangbo.lu@nxp.com>