irqdomain: Make of_phandle_args_to_fwspec() generally available

of_phandle_args_to_fwspec() can be generally useful to code extracting a DT
of_phandle and using an irq_fwspec to use the hierarchical irqdomain API.

Make it visible to the rest of the kernel, including modules.

Link: https://lore.kernel.org/r/20210929163847.2807812-2-maz@kernel.org
Tested-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>