pwm: Drop of_pwm_simple_xlate() in favour of of_pwm_xlate_with_flags()

Since the previous commit the latter function can do everything that the
former does. So simplify accordingly.

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