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>

MAINTAINERS

@@ -13086,6 +13086,7 @@ F:	drivers/video/backlight/pwm_bl.c
 F:	include/linux/pwm_backlight.h
 F:	drivers/gpio/gpio-mvebu.c
 F:	Documentation/devicetree/bindings/gpio/gpio-mvebu.txt
+K:	pwm_(config|apply_state|ops)
 
 PXA GPIO DRIVER
 M:	Robert Jarzmik <robert.jarzmik@free.fr>