Mikrotik built six LEDs into the Routerboard 532, from which one is
destined for custom use, the so called "User LED". This patch adds a
driver for it, based on the LEDs class.

Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>