powerpc64: Increase the nap level on power9 idling

The POWER9 documentation specifies that levels 0-3 are the 'lightest' sleep
level, meaning lowest latency and with no state loss. However, state 3 is
not implemented, and is instead reserved for future chips. This now
properly configures the PSSCR, specifying state 2 as the lowest level to
enter, but request level 0 for quickest sleep level. If the OCC determines
that the CPU can enter states 1 or 2 it will trigger the transition to those
states on demand.

MFC after: 1 week