net: phy: fixed-phy: Drop GPIO from fixed_phy_add()

All users of the fixed_phy_add() pass -1 as GPIO number
to the fixed phy driver, and all users of fixed_phy_register()
pass -1 as GPIO number as well, except for the device
tree MDIO bus.

Any new users should create a proper device and pass the
GPIO as a descriptor associated with the device so delete
the GPIO argument from the calls and drop the code looking
requesting a GPIO in fixed_phy_add().

In fixed phy_register(), investigate the "fixed-link"
node and pick the GPIO descriptor from "link-gpios" if
this property exists. Move the corresponding code out
of of_mdio.c as the fixed phy code anyways requires
OF to be in use.

Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>