leds: add SGI IP30 led support

This patch implemenets a driver to support the front panel LEDs of
SGI Octane (IP30) workstations.

Reviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>