leds: MAINTAINERS: include dt-bindings headers

Include the Devicetree binding headers in LED SUBSYSTEM entry.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>