iio:adc:ti-adc081c: Drop of_match_ptr and change to mod_devicetable.h

Whilst this driver already supports explicit ACPI bindings we
might as well also allow for PRP0001 based binding.

I'm also keen to remove of_match_ptr from IIO drivers to avoid
this (now) anti-pattern getting coppied into new drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>