pwm: lpss: Allow other drivers to enable PWM LPSS

The PWM LPSS device can be embedded in another device.
In order to enable it, allow that drivers to probe
a corresponding device.

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