iio: adc: ad7124: Switch from of specific to fwnode based property handling

iio: adc: ad7124: Switch from of specific to fwnode based property handling

Using the generic firmware data access functions from property.h
provides a number of advantages:
 1) Works with different firmware types.
 2) Doesn't provide a 'bad' example for new IIO drivers.
 3) Lets us use the new _scoped() loops with automatic reference count
    cleanup for fwnode_handle

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240218172731.1023367-4-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>