iio:adc:axp20x: Convert from OF to generic fw / device properties

Whilst fairly unlikely anyone will ever use this driver with anything
other than DT, we are trying to move IIO over to the generic interfaces
where easy to do so.

In this case this involved moving to generic check on presence
of fwnode, generic device_get_match_data() and dropping the of_match_ptr
protection.  Also relevant header changes to have property.h and
mod_devicetable.h only.

Also drop the casting away of a const in favour of retaining
the const throughout.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>