target/arm: Move start-powered-off property to generic CPUState

There are other platforms which also have CPUs that start powered off, so
generalize the start-powered-off property so that it can be used by them.

Note that ARMv7MState also has a property of the same name but this patch
doesn't change it because that class isn't a subclass of CPUState so it
wouldn't be a trivial change.

This change should not cause any change in behavior.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Message-Id: <20200826055535.951207-2-bauerman@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

exec.c

@@ -899,6 +899,7 @@ Property cpu_common_props[] = {
     DEFINE_PROP_LINK("memory", CPUState, memory, TYPE_MEMORY_REGION,
                      MemoryRegion *),
 #endif
+    DEFINE_PROP_BOOL("start-powered-off", CPUState, start_powered_off, false),
     DEFINE_PROP_END_OF_LIST(),
 };