workaround Intel Braswell/Cherry Trail mwait hang

workaround Intel Braswell/Cherry Trail mwait hang

dlg has a Dell Wyse 3040 with
cpu0: Intel(R) Atom(TM) x5-Z8350 CPU @ 1.44GHz, 480.02 MHz, 06-4c-04
cpu0: mwait min=64, max=64, C-substates=0.2.0.0.0.0.3.3, IBE
which hangs soon after the login prompt with MP kernels

This is a hardware bug described in:
Intel Atom Z8000 Processor Series Specification Update
Document Number: 332067-012
"CHT45 Processor May Not Wake From C6 or Deeper Sleep State"

tested by dlg@, ok guenther@