ppc/pnv: Fix direct controls quiesce

ppc/pnv: Fix direct controls quiesce

powernv CPUs have a set of control registers that can stop, start, and
do other things to control a thread's execution.

Using this interface to stop a thread puts it into a particular state
that can be queried, and is distinguishable from other things that might
stop the CPU (e.g., going idle, or being debugged via gdb, or stopped by
the monitor).

Add a new flag that can speficially distinguish this state where it is
stopped with control registers. This solves some hangs when rebooting
powernv machines when skiboot is modified to allow QEMU to use the CPU
control facility (that uses controls to bring all secondaries to a known
state).

Fixes: c8891955086 ("ppc/pnv: Implement POWER10 PC xscom registers for direct controls")
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>