drivers: mdio: add bit-bang driver

Add MDIO driver that works through GPIO pins. This driver is
useful when a microcontroller doesn't have MDIO bus or when multiple
separate MDIO buses are required. The driver provides access to
the MDIO bus through GPIO pins for any SoC that has GPIO pin control
available.

Signed-off-by: Aleksandr Senin <al@meshium.net>