of/irq: make of_irq_find_parent static

of_irq_find_parent has no users outside of of_irq.c, so it does not make
sense to expose it in of_irq.h. Therefore remove the prototype and dummy
implmeentation and make the function static instead.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Rob Herring <robh@kernel.org>