MAINTAINERS: add entry for the PHY library

MAINTAINERS: add entry for the PHY library

The PHY library has been subject to some changes, new drivers and DT
interactions over the past few months. Add myself as a maintainer for
the core PHY library parts and drivers. Make sure the PHY library entry
also covers the Device Tree files which have a close interaction with
the MDIO bus, PHY connection and Ethernet PHY mode parsing.

CC: Grant Likely <[email protected]>
CC: Shaohui Xie <[email protected]>
CC: Andy Fleming <[email protected]>
Signed-off-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>