net: phy: realtek: Add support for PHY LEDs on RTL8211F

Realtek RTL8211F Ethernet PHY supports 3 LED pins which are used to
indicate link status and activity. Add minimal LED controller driver
supporting the most common uses with the 'netdev' trigger.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>