pwm: Provide wrappers for storing and getting driver private data

These functions are useful to store and query driver private data
depending on the pwm_chip. After struct pwm_chip got its own struct
device, this can make use of dev_get_drvdata() and dev_set_drvdata() on
that device. These functions are required already now to convert
drivers to pwmchip_alloc() which must happen before changing
pwm_chip::dev.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/67514cdf29d29bd8b4ad8d44fac87f6ae6dca1e5.1707900770.git.u.kleine-koenig@pengutronix.de


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