of/irq: of_irq_init: add check for parent equal to child node

With the revert of "of/irq: of_irq_find_parent: check for parent equal to
child" (dc937280), we need another way to handle parent node equal
to the child node. This can simply be handled in of_irq_init by checking
for this condition.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Tested-by: Pawel Moll <pawel.moll@arm.com>
Tested-by: Stephen Warren <swarren@nvidia.com>