of: OF_IRQ should depend on IRQ_DOMAIN

If CONFIG_IRQ_DOMAIN=n:

drivers/of/irq.c: In function ‘of_irq_get’:
drivers/of/irq.c:406: error: implicit declaration of function ‘irq_find_host’
drivers/of/irq.c:406: warning: assignment makes pointer from integer without a cast
make[2]: *** [drivers/of/irq.o] Error 1

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Rob Herring <robh@kernel.org>