leds: ariel: Add driver for status LEDs on Dell Wyse 3020

This adds support for controlling the LEDs attached to the Embedded
Controller on a Dell Wyse 3020 "Ariel" board.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Pavel Machek <pavel@ucw.cz>