iio: st_accel: use ACPI orientation data

Platform-specific ST accelerometer mount matrix information can be
provided by returning a package of 6 integers from the ACPI _ONT
method. This has been seen on Acer products such as Veriton Z4860G,
Z6860G and A890, which include a ST SMO8840 sensor. We have also
confirmed experimentally that the Windows driver uses such information.

The _ONT data format was explained by a ST vendor contact. However,
strangely enough, the _ONT transformations must be applied after first
applying another mount matrix which we determined experimentally. ST
have not commented on why this is the case, but we imagine that perhaps
earlier devices (before _ONT was introduced) required this translation
and hence it became 'standard.'

Interpret the _ONT data and export the equivalent mount matrix to
userspace.

If no _ONT data is present, no mount matrix is exported.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>