of/irq: Create stub for of_irq_find_parent when !CONFIG_OF

of_irq_find_parent is a handy function to use outside the confines of
the Open Firmware subsystem. One such use-case is when the IRQ Domain
wishes to find an IRQ domain for a given device node. Currently it can
not take any notice of the 'interrupt-parent' property. Instead it
just uses the first IRQ controller as it climbs the Device Tree. If
we were to use this as a precursor the resultant controller is more
likely to be correct.

Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>