nitrogen6x: forcibly hang at power-off

When instructed to "power off", we currently simply re-boot,
which doesn't allow someone an easy way to shut down a system
before pulling power.

This patch forces a stall in the kernel at the point where we
would disconnect power if we had such control.

The Android U/I doesn't provide an indication of when it's
safe to unplug power, so users will need to be instructed
to wait for the spinning-thing to stop spinning.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>