leds: initial support for Turris Omnia LEDs

This adds basic support for LEDs on the front side of CZ.NIC's Turris
Omnia router.

There are 12 RGB LEDs. The controller supports HW triggering mode for
the LEDs, but this driver does not support it yet, and sets all the LEDs
defined in device-tree into SW mode upon probe.

This driver uses the multicolor LED framework.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>