Parse interrupts from device tree properly

[axel-h]

Follow-u up from #79

Drop the best guess in the interrupt parsing and read the #interrupt-cells from the interrupt controller instead. Use the device tree parser we already use for the kernel.
See #79 (review)

The camkes-tool already gets given an import path to the capDL python library,
so it shouldn't be too hard to give it access to the kernel hardware_gen python
lib that would allow proper irq parsing.