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 <grant.likely@linaro.org>
CC: Shaohui Xie <shaohui.xie@freescale.com>
CC: Andy Fleming <afleming@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>