dev-arm: Add doorbell interface class

JIRA: https://gem5.atlassian.net/browse/GEM5-768

Change-Id: I0d264a74cbf8ca0f780314ad01fb0dd0765a0464
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34377
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/arm/SConscript

@@ -40,6 +40,7 @@ Import('*')
 if env['TARGET_ISA'] == 'arm':
     SimObject('AbstractNVM.py')
     SimObject('Display.py')
+    SimObject('Doorbell.py')
     SimObject('FlashDevice.py')
     SimObject('GenericTimer.py')
     SimObject('Gic.py')