pwm: sprd: Drop duplicated tracking of the parent device

The pwmchip stores a pointer to the parent device, so there is no need
to store another copy in driver private data. Drop struct
sprd_pwm_chip::dev and use the pwm_chip's parent pointer instead.

Tested-by: Chunyan Zhang <zhang.lyra@gmail.com>
Link: https://lore.kernel.org/r/f85771f4bf659c0fdee30cf117b87fd877bad5e4.1707900770.git.u.kleine-koenig@pengutronix.de


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