pwm: Drop support for legacy drivers

There are no drivers left providing the legacy callbacks. So drop
support for these.

If this commit breaks your out-of-tree pwm driver, look at e.g. commit
ec00cd5e ("pwm: renesas-tpu: Implement .apply() callback") for an
example of the needed conversion for your driver.

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