net: phy: at803x: add device tree binding

Add support for configuring the CLK_25M pin as well as the RGMII I/O
voltage by the device tree.
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>