iio: accel: st_accel: Add support for the SMO8840 ACPI id

The Lenovo Ideapad Miix 320 uses an st accelerometer with an ACPI id
of SMO8840, add support for this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>