pwm: Rearrange structures to group members by purpose

In pwm_ops there are a few callbacks that are not supposed to be used by
new drivers. Group them at the end of the structure and add a comment.

Similarily for struct pwm_chip group the members that drivers shouldn't
care about at the end and mark them as internal with another comment.

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