MAINTAINERS: Add additional co-maintainer to LEDs

Add myself as co-maintainer for the LED subsystem,
in support of Pavel during busy times.

Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Lee Jones <lee@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>