This prepares the driver for further changes that will drop struct
pwm_chip chip from struct img_pwm_chip. Use the pwm_chip as driver data
instead of the img_pwm_chip to get access to the pwm_chip in
img_pwm_remove() and the PM callbacks without using imgchip->chip.

Link: https://lore.kernel.org/r/2ddf76d127f69eca7d9faa6475091e432e890ac9.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>