arch-arm: Add recursion for DTB entry generation inside ArmISA

In order to generate the ArmPMU's DTB entry, we have to enable recursion
from the ArmISA.

This commit follows this mailing list entry:
https://www.mail-archive.com/gem5-users@gem5.org/msg18401.html

Change-Id: I73012755f0f8c8d4d17278793cf16cb1e8b011df
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35555
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/ArmISA.py

@@ -36,6 +36,7 @@
 from m5.params import *
 from m5.proxy import *
 
+from m5.SimObject import SimObject
 from m5.objects.ArmPMU import ArmPMU
 from m5.objects.ArmSystem import SveVectorLength
 from m5.objects.BaseISA import BaseISA
@@ -124,3 +125,7 @@ class ArmISA(BaseISA):
     # allocated, instead of an ArmSystem
     sve_vl_se = Param.SveVectorLength(1,
         "SVE vector length in quadwords (128-bit), SE-mode only")
+
+    # Recurse into subnodes to generate DTB entries. This is mainly needed to
+    # generate the PMU entry.
+    generateDeviceTree = SimObject.recurseDeviceTree