pwm: pxa: Don't reimplement of_device_get_match_data()

Apart from the return type pxa_pwm_get_id_dt() reimplements
of_device_get_match_data(). Drop the former and replace the call to it
by the latter.

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