irqdomain: Add forward declaration of fwnode_handle

irqdomain.h is a user of struct fwnode_handle. Add forward declaration of it.

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/20201030165919.86234-3-andriy.shevchenko@linux.intel.com