ACPI / LPSS: Add missing prv_offset setting for byt/cht PWM devices

The LPSS PWM device on on Bay Trail and Cherry Trail devices has a set
of private registers at offset 0x800, the current lpss_device_desc for
them already sets the LPSS_SAVE_CTX flag to have these saved/restored
over device-suspend, but the current lpss_device_desc was not setting
the prv_offset field, leading to the regular device registers getting
saved/restored instead.

This is causing the PWM controller to no longer work, resulting in a black
screen,  after a suspend/resume on systems where the firmware clears the
APB clock and reset bits at offset 0x804.

This commit fixes this by properly setting prv_offset to 0x800 for
the PWM devices.

Cc: stable@vger.kernel.org
Fixes: e1c74817 ("ACPI / LPSS: Add Intel BayTrail ACPI mode PWM")
Fixes: 1bfbd8eb

 ("ACPI / LPSS: Add ACPI IDs for Intel Braswell")
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>