MAINTAINERS: LEDs: Add designated reviewer for LED subsystem

Add a designated reviewer for the LED subsystem as there
are already two maintainers assigned.

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