pwm: lpc18xx: Implement .apply() callback

To eventually get rid of all legacy drivers convert this driver to the
modern world implementing .apply().
This pushes a variant of pwm_apply_legacy into the driver that was slightly
simplified because the .set_polarity callback was a noop.

There is no change in behavior.

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