igc: Export LEDs

Each i225 has three LEDs. Export them via the LED class framework.

Each LED is controllable via sysfs. Example:

$ cd /sys/class/leds/igc_led0
$ cat brightness      # Current Mode
$ cat max_brightness  # 15
$ echo 0 > brightness # Mode 0
$ echo 1 > brightness # Mode 1

The brightness field here reflects the different LED modes ranging
from 0 to 15.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>