pwm: lpss: Use DEFINE_RUNTIME_DEV_PM_OPS() and pm_ptr() macros

Using these new macros allows the compiler to remove the unused dev_pm_ops
structure and related functions if !CONFIG_PM without the need to mark
the functions __maybe_unused.

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