sim-power: Specify the states a PowerState object can be in

This commit adds the concept of possible power states to the
PowerState SimObject. This is a list of the power states a specific
object can be in. Before transitioning to a power state, a PowerState
object will first check if the requested power states is actually an
allowed state. The user can restricted the power states a
ClockedObject can go to during configuration. In addition, this change
sets the power states, a CPU can be in.

Change-Id: Ida414a87554a14f09767a272b54b5d19bfc8e911
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28050
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/BaseCPU.py

@@ -303,3 +303,7 @@ class BaseCPU(ClockedObject):
             cpus_node.append(node)
 
         yield cpus_node
+
+    def __init__(self, **kwargs):
+        super(BaseCPU, self).__init__(**kwargs)
+        self.power_state.possible_states=['ON', 'CLK_GATED', 'OFF']