MAINTAINERS: Add include/dt-bindings/pwm to PWM SUBSYSTEM

Maintainers of the directory Documentation/devicetree/bindings/pwm
are also the maintainers of the corresponding directory
include/dt-bindings/pwm.

Add the file entry for include/dt-bindings/pwm to the appropriate
section in MAINTAINERS.
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

MAINTAINERS

@@ -16199,6 +16199,7 @@ F:	Documentation/driver-api/pwm.rst
 F:	drivers/gpio/gpio-mvebu.c
 F:	drivers/pwm/
 F:	drivers/video/backlight/pwm_bl.c
+F:	include/dt-bindings/pwm/
 F:	include/linux/pwm.h
 F:	include/linux/pwm_backlight.h
 K:	pwm_(config|apply_state|ops)