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: NRob Herring <rob.herring@calxeda.com>
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

include/linux/of_irq.h

@@ -83,6 +83,11 @@ static inline unsigned int irq_of_parse_and_map(struct device_node *dev,
 {
 	return 0;
 }
+
+static inline void *of_irq_find_parent(struct device_node *child)
+{
+	return NULL;
+}
 #endif /* !CONFIG_OF */
 
 #endif /* __OF_IRQ_H */