pwm: sysfs: Switch to DEFINE_SIMPLE_DEV_PM_OPS() and pm_sleep_ptr()

Using these newer macros allows the compiler to remove the unused
structure and functions when !CONFIG_PM_SLEEP + removes the need to
mark pm functions __maybe_unused.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>