irqdomain: Make __irq_domain_add() less OF-dependent

__irq_domain_add() relies in some places on the fact that the fwnode
can be only of type OF. This prevents refactoring of the code to support
other types of fwnode. Make it less OF-dependent by switching it
to use the fwnode directly where it makes sense.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200520164927.39090-1-andriy.shevchenko@linux.intel.com