arch-arm, dev-arm: Autogen PSCI node in DTB

This is controlled via the python only _have_psci parameter
This flag will be checked when auto-generarting a PSCI node. A client
(e.g Linux) would then be able to know if it can use the PSCI APIs

Change-Id: I16c4a67bd358eca3dfff6c98ab8a602a31e1c751
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27387
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/ArmSystem.py

@@ -1,4 +1,4 @@
-# Copyright (c) 2009, 2012-2013, 2015-2019 ARM Limited
+# Copyright (c) 2009, 2012-2013, 2015-2020 ARM Limited
 # All rights reserved.
 #
 # The license below extends only to copyright in the software and shall
@@ -83,6 +83,12 @@ class ArmSystem(System):
         "Base of the 64KiB PA range used for memory-mapped m5ops. Set to 0 "
         "to disable.")
 
+    # Set to true if simulation provides a PSCI implementation
+    # This flag will be checked when auto-generating
+    # a PSCI node. A client (e.g Linux) would then be able to
+    # know if it can use the PSCI APIs
+    _have_psci = False
+
     def generateDtb(self, filename):
         """
         Autogenerate DTB. Arguments are the folder where the DTB