pwm: ab8500: Implement .apply instead of .config, .enable and .disable

To eventually get rid of all legacy drivers convert this driver to the
modern world implementing .apply().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>