powerpc/powernv: Add a driver for the POWER9 XIVE interrupt controller

The XIVE (External Interrupt Virtualization Engine) is a new interrupt
controller present in IBM's POWER9 processor. It's a very powerful,
very complex device using queues and shared memory to improve interrupt
dispatch performance in a virtualized environment.

This yields a ~10% performance improvment over the XICS emulation mode,
measured in both buildworld, and 'dd' from nvme to /dev/null.

Currently, this only supports native access.

MFC after: 1 month