This prepares the driver for further changes that will drop struct
pwm_chip chip from struct pwm_sifive_ddata. Use the pwm_chip as driver
data instead of the pwm_sifive_ddata to get access to the pwm_chip in
pwm_sifive_remove() without using ddata->chip. In the clock rate
notifier it's not possible to get the pwm_chip without adding a pointer
to this to struct pwm_sifive_ddata. Instead of that add a parent device
pointer which is all that is needed there.

Link: https://lore.kernel.org/r/b7b7985f4dc746f6a36c5048d428c4ed0a2d42dc.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>