Add an interrupt controller

[erlingrj]

It would be great to support several interrupt sources like:

• Any GPIO pin
• NoC event
• Events from devices attached to the bus

Currently there is a single wire per thread carrying an interrupt signal. We could develop a module which allows a thread to configure/MUX different interrupt sources onto this wire. After an interrupt occurs the thread would need to read from this module which sources that asserted the interrupt line.