pwm: Implement some checks for lowlevel drivers

There are some expectations which the callbacks provided by lowlevel
drivers should fulfill. Implement checks that help driver authors to get
these semantics right. As these have some overhead the checks can be
disabled using a Kconfig setting.

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