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>

MAINTAINERS

@@ -13078,6 +13078,7 @@ M:	Thierry Reding <thierry.reding@gmail.com>
 L:	linux-pwm@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/thierry.reding/linux-pwm.git
+Q:	https://patchwork.ozlabs.org/project/linux-pwm/list/
 F:	Documentation/driver-api/pwm.rst
 F:	Documentation/devicetree/bindings/pwm/
 F:	include/linux/pwm.h