net: phy: mdio-gpio: Add #defines for the GPIO index's

The GPIOs are described in device tree using a list, without names.
Add defines to indicate what each index in the list means. These
defines should also be used by platform devices passing GPIOs via a
GPIO lookup table.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>