phy: mdio: add kerneldoc for __devm_mdiobus_register()

This function is not documented. Add a short kerneldoc description.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>