Adding cpuidle modules and refactoring Device cpufreq APIs.

cpuidle module implements cpuidle state discovery, query and
manipulation for a Linux device. This replaces the more primitive
get_cpuidle_states method of LinuxDevice.

Renamed APIs (and added a couple of new ones) to be more consistent:

"core" APIs take a core name as the parameter (e.g. "a15") or whatever
is listed in core_names for that device.
"cluster" APIs take a numeric cluster ID (eg. 0) as the parameter. These
get mapped using core_clusters for that device.
"cpu" APIs take a cpufreq cpu ID as a parameter. These could be
integers, e.g. 0, or full string id, e.g. "cpu0".

wlauto/devices/android/juno/__init__.py

@@ -42,7 +42,7 @@ class Juno(BigLittleDevice):
 
     has_gpu = True
 
-    modules = [
+    core_modules = [
         'vexpress',
     ]
 
@@ -143,9 +143,6 @@ class Juno(BigLittleDevice):
         # perfrom a ahard reset instead
         self.hard_reset()
 
-    def get_cpuidle_states(self, cpu=0):
-        return {}
-
     def hard_reset(self):
         self.disconnect()
         self.adb_name = None  # Force re-acquire IP address on reboot. pylint: disable=attribute-defined-outside-init