of: Add dummy of_irq_to_resource_table() for IRQ_OF=n

If CONFIG_IRQ_OF=n:

drivers/built-in.o: In function `of_device_alloc':
(.text+0x72bce): undefined reference to `of_irq_to_resource_table'
make: *** [vmlinux] Error 1

of_device_alloc() calls of_irq_to_resource_table() with nr_irqs = 0 due
to of_irq_count() already being a dummy, so just add a dummy for
of_irq_to_resource_table(), too.

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