of/irq: Export of_irq_find_parent again

of_irq_find_parent was made static since it had no users outside of
of_irq.c. Export it again since we are going to use it again.

Signed-off-by: Carlo Caione <carlo@endlessm.com>
[robh: move of_irq_find_parent to correct ifdef section]
Signed-off-by: Rob Herring <robh@kernel.org>