pwm: tiecap: 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 ecap_pwm_chip. To just not have to
do that, rework ecap_pwm_save_context() and ecap_pwm_restore_context
take a pwm_chip. Also use the pwm_chip as driver data instead of the
ecap_pwm_chip.

Link: https://lore.kernel.org/r/ed031f201ff52c6b298de2dc81b06aad3a0207f8.1707900770.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>