pwm: Let the of_xlate callbacks accept references without period

With this extension of_pwm_xlate_with_flags() is suitable to replace the
custom xlate function of the pwm-clps711x driver.

While touching these very similar functions align their implementations.

Link: https://lore.kernel.org/r/127622315d07d9d419ae8e6373c7e5be7fab7a62.1704835845.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>