MAINTAINERS: Add patchwork link for PWM entry

This instance collects patches and Thierry updates the patches' status
there, so I consider it used and suitable to document it officially.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>