Don't wrap the declaration of gpio_alloc_intr_resource() in #ifdef INTRNG,
wrap the implementation so that it returns an error if INTRNG support is
not available. It should be possible to write a non-INTRNG implementation
of this function some day. In the meantime, there is code that contains
calls to this function (so the decl is needed), but have runtime checks to
avoid calling it in the non-INTRNG case.