pwm: tiehrpwm: Change prototype of helpers to prepare further changes

pwm: tiehrpwm: Change prototype of helpers to prepare further changes

This prepares the driver for further changes that will make it harder to
determine the pwm_chip from a given ehrpwm_pwm_chip. To just not have to
do that, rework ehrpwm_pwm_save_context() and
ehrpwm_pwm_restore_context() take a pwm_chip. Also use the pwm_chip as
driver data instead of the ehrpwm_pwm_chip.

Link: https://lore.kernel.org/r/79052207cdf71f0882ae13fe1a192ef6f6dba35b.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>