To answer the actual question that I asked:

In this case, when I switched the line interrupts = <28 3> to interrupts = <28 8>, the IRQ was successfully mapped. (My device still doesn't work, because that's not actually the correct interrupt line, but I'll deal with that separately).

The actual change (3 vs 8) is changing from IRQ_TYPE_EDGE_BOTH to IRQ_TYPE_LEVEL_LOW - I guess this particular GPIO driver (tegra194-gpio-aon) doesn't support edge-triggering?