net: stmmac: drop the reset GPIO from struct stmmac_mdio_bus_data

No platform uses the "reset_gpio" field from stmmac_mdio_bus_data
anymore. Drop it so we don't get any new consumers either.

Plain GPIO numbers are being deprecated in favor of GPIO descriptors. If
needed any new non-OF platform can add a GPIO descriptor lookup table.
devm_gpiod_get_optional() will find the GPIO in that case.

Suggested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>