acpi: lpss: call pwm_add_table() for BSW PWM device

On x86 we do not have devicetree to link the PWM controller and
the display controller together. So someone needs to call
pwm_add_table() to create the link, so that the i915 driver's
pwm_get(dev, "pwm_backlight") call returns the lpss' pwm0.

The PWM subsystem does not want to have pwm_add_table() calls
directly in PWM drivers (this leads to probe ordering issues),
so lets do it here since the acpi-lpss code is always builtin.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>