iio: adc: hx711: Switch from of specific to fwnode property handling.

Allows driver to be used with other firmware types and removes an
example that might be copied into new IIO drivers.

Cc: Andreas Klinger <ak@it-klinger.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240218172731.1023367-9-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>