Add an IPMI attachment for PowerNV systems

IPMI access on PowerNV systems is done through the OPAL firmware. This adds a
simple attachment for communicating with the FSP/BMC on these machines. This
has been tested on a Talos POWER9 workstation, only in the bootup phase, noting
the successful attachment messages:

...
ipmi0: IPMI device rev. 0, firmware rev. 2.00, version 2.0, device support mask 0
ipmi0: Number of channels 2
...

The ipmi device has not been added to GENERIC64, but may be after further
testing. It may also eventually be added to the ipmi module at that point.