MAINTAINERS: Add phylink keyword to SFF/SFP/SFP+ MODULE SUPPORT

Russell king maintains phylink, as part of the SFP module support.
However, much of the review work is about drivers swapping from phylib
to phylink. Such changes don't make changes to the phylink core, and
so the F: rules in MAINTAINERS don't match. Add a K:, keywork rule,
which hopefully get_maintainers will match against for patches to MAC
drivers swapping to phylink.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>