MAINTAINERS: Remove myself as PWM maintainer

Thierry and Uwe are doing a fine job, leaving me surplus to requirement.

Happy to pop back on-board if anything changes in the future.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: linux-pwm@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>