MAINTAINERS: Add a selection of PWM related keywords to the PWM entry

This is just a small subset of the relevant functions, but should at
least catch all new code as every consumer has to call pwm_apply_state()
(or the legacy function pwm_config()) and every PWM provider has to
implement pwm_ops.

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