pwm: sprd: Provide a helper to cast a chip to driver data

Similar to most other PWM drivers provide a static inline function to
calculate driver data from a given pwmchip.

Link: https://lore.kernel.org/r/20230929161918.2410424-8-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>