leds: Add user LED driver for NIC78bx device

Add the driver to support User LEDs on PXI Embedded Controller.

Signed-off-by: Hui Chun Ong <hui.chun.ong@ni.com>
Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>