irqdomain: Replace open coded of_node_to_fwnode()

of_node_to_fwnode() should be used for conversion.  Replace the open coded
variant of it in of_phandle_args_to_fwspec().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20201030165919.86234-4-andriy.shevchenko@linux.intel.com