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

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>