net: phy: realtek: Add support for LED configuration on RTL8211F

Add support for reading the LED configuration for the (up to) three
LEDs from device-tree for the Realtek RTL8211F PHY.

The register layout for this is very simple:
- 10Mbit/s link at LCR[0,5,10] for LED{0,1,2}
- 100Mbit/s link at LCR[1,6,11] for LED{0,1,2}
- 1000Mbit/s link at LCR[3,8,13] for LED{0,1,2}
- RX/TX activity at LCR[4,9,14] for LED{0,1,2}
- EEE at EEELCR[1,2,3] for LED{0,1,2}

Re-configure the LEDs only when specified in device-tree, otherwise keep
the configuration set using pin-strapping or the settings from the
bootloader.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>