I think I addressed this on the other email with the actual driver.
Anyway, I thought that this is pretty much the dummy irqchip code is
for; I have seen some other drivers do the same thing. It is true that
this is a really basic "interrupt controller;" it cannot mask on its
own, etc; nevertheless, I think you will pretty much end up with the
same code for an "I2C controller;" it just won't use an irq_domain.