net: of_mdio: parse "max-speed" property to set PHY supported features

The "max-speed" property is defined per the ePAPR specification to
express the maximum speed a PHY supports. Use that property, if present
to set the phydev->supported features which properly restricts the PHY
within the range of defined speeds.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>