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- When the SCPI shutdown/reset command returns success, the SCP is
  still waiting for the CPU to enter WFI. Do that.
- Peform board-level poweroff before CPU poweroff. If there is a PMIC
  available, it will turn everything off including the CPUs, so doing
  CPU poweroff first is a waste of cycles.
- During poweroff, attempt to turn off the local CPU using the ARISC.
  This should use slightly less power than just an infinite WFI.
- Drop the WFI in the reset failure path. The panic will hang anyway.

Change-Id: I897efecb3fe4e77a56041b97dd273156ec51ef8e
Signed-off-by: Samuel Holland <samuel@sholland.org>