MAINTAINERS: pwm: Thierry steps down, Uwe takes over

It's not easy to let go responsibility for a subsystem that one cared
for for a long time, but Thierry realized that his heart isn't in the
pwm framework any more.

Thierry cared for the pwm subsystem (commit 200efedd ("pwm: Take
over maintainership of the PWM subsystem")) as a maintainer during
nearly 12 years. A big thanks for the time, effort and dedication spend
during that time.

Uwe takes over maintenance.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>