iio: core: Convert to use firmware node handle instead of OF node

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