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: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>