Implement powerdown. This involves writing a magic bit somewhere in the

Implement powerdown.  This involves writing a magic bit somewhere in the
address space of the SPMI PMU to prevent the machine from immediately
starting up again.

The implementaton makes aplpmu(4) provide powerdownfn(), which sets the
magic bit and then chains into cpuresetfn().  It also makes aplsmc(4)
provide cpuresetfn() to reset the machine via the SMC.  Resetting via
the watchdog works as well (and will powerdown the machine if the magic
bit is set) but letting the SMC handle things might do some other
required steps.

ok patrick@