irqdomain: Allow software nodes for IRQ domain creation

In some cases we need to have an IRQ domain created out of software node.

One of such cases is DesignWare GPIO driver when it's instantiated from
half-baked ACPI table (alas, we can't fix it for devices which are few years
on market) and thus using software nodes to quirk this. But the driver
is using IRQ domains based on per GPIO port firmware nodes, which are in
the above case software ones. This brings a warning message to be printed

  [   73.957183] irq: Invalid fwnode type for irqdomain

and creates an anonymous IRQ domain without a debugfs entry.

Allowing software nodes to be valid for IRQ domains rids us of the warning
and debugs gets correctly populated.

  % ls -1 /sys/kernel/debug/irq/domains/
  ...
  intel-quark-dw-apb-gpio:portA

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[maz: refactored commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200520164927.39090-3-andriy.shevchenko@linux.intel.com