net: phy: Add PHY driver for mv88e61xx switches

The previous mv88e61xx driver was a driver for configuring the
switch, but did not integrate with the PHY/networking system, so
it could not be used as a PHY by U-boot.  This is a complete
rework to support this device as a PHY.

Signed-off-by: Kevin Smith <kevin.smith@elecsyscorp.com>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>