pwm: Add might_sleep() annotations for !CONFIG_PWM API functions

The normal implementations of these functions make use of mutexes. To
make it obvious that these functions might sleep also add annotations to
the dummy implementations in the !CONFIG_PWM case.

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