leds: Add rb532 LED driver for the User LED

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>