net: fixed-phy: Add fixed_phy_register_with_gpiod() API

Add fixed_phy_register_with_gpiod() API. It lets users create a
fixed_phy instance that uses a GPIO descriptor which was obtained
externally e.g. through platform data.
This enables platform devices (non-DT based) to use GPIOs for link
status.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>