MAINTAINERS: Add entry for stable backlight sysfs ABI documentation

Add an entry for the stable backlight sysfs ABI to the MAINTAINERS
file.
Signed-off-by: NMatthias Kaehlcke <mka@chromium.org>
Acked-by: NDaniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

MAINTAINERS

@@ -2906,6 +2906,7 @@ F:	drivers/video/backlight/
 F:	include/linux/backlight.h
 F:	include/linux/pwm_backlight.h
 F:	Documentation/devicetree/bindings/leds/backlight
+F:	Documentation/ABI/stable/sysfs-class-backlight
 
 BATMAN ADVANCED
 M:	Marek Lindner <mareklindner@neomailbox.ch>