This prepares the driver for further changes that will drop struct
pwm_chip chip from struct rz_mtu3_pwm_chip. Use the pwm_chip as driver
data and devm callback parameter instead of the rz_mtu3_pwm_chip to get
access to the pwm_chip in various functions without using
rz_mtu3_pwm->chip.

Link: https://lore.kernel.org/r/707f2aced94a235f244022f10202ce7730b1168b.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>