PowerNV: Put processor to power-save state in idle thread

When processor enters power-save state it releases resources shared with other
cpu threads which makes other cores working much faster.

This patch also implements saving and restoring registers that might get
corrupted in power-save state.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Reviewed by: jhibbits, nwhitehorn, wma
Sponsored by: IBM, QCM Technologies
Differential revision: https://reviews.freebsd.org/D14330