iio:adc:ti-adc084s021: drop of_match_ptr protection

Allows ACPI PRP0001 to be used with this driver.
Also adds mod_devicetable.h header as the driver is directly using
struct of_device_id which is defined in that file.

I'm also trying to clear out of_match_ptr use in IIO to avoid it
being cut and paste into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Mårten Lindahl <martenli@axis.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>