of: unittest: Switch to use fwnode instead of of_node

The OF node in the GPIO library is deprecated and soon will be removed.
GPIO library now accepts fwnode as a firmware node, so switch the module
to use it instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220708214539.7254-1-andriy.shevchenko@linux.intel.com