pwm: img: Implement .apply() callback

To eventually get rid of all legacy drivers convert this driver to the
modern world implementing .apply(). This just pushes down a slightly
optimized variant of how legacy drivers are handled in the core.

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