iio: adc: ti-ads7950: Add OF device ID table

The driver doesn't have a struct of_device_id table but supported devices
are registered via Device Trees. This is working on the assumption that a
SPI device registered via OF will always match a legacy SPI device ID and
that the MODALIAS reported will always be of the form spi:<device>.

There is an ACPI method to enumerate such devices via specific ACPI ID
and use of compatible strings. It will not work for the drivers which
have no OF match ID table present.

Besides this could change in the future so the correct approach is to
have an OF device ID table if the devices are registered via OF.

Tested-by: David Lechner <david@lechnology.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>