irqdomain: Get rid of special treatment for ACPI in __irq_domain_add()

Now that __irq_domain_add() is able to better deals with generic
fwnodes, there is no need to special-case ACPI anymore.

Get rid of the special treatment for ACPI.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200520164927.39090-2-andriy.shevchenko@linux.intel.com