Switch the IIO core to use firmware node handle instead of OF node.
This will allow to get label from firmware on non-OF systems.

Note, this doesn't change of_iio_*() APIs for now.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220413180202.19220-1-andriy.shevchenko@linux.intel.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>