iio: adc: fsl-imx25-gcq: Switch from of specific handing to fwnode based.

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

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240218172731.1023367-2-jic23@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>