dev-arm: fix --generate-dtb for ARM

Was failing with:

NameError: global name 'FdtNode' is not defined

The problem was introduced at: 75831ce5b7

Change-Id: I7e2ce0e5311e7814229945b9f4e7318a8652dc1f
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15875
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/dev/arm/Gic.py

@@ -37,6 +37,7 @@
 
 from m5.params import *
 from m5.proxy import *
+from m5.util.fdthelper import *
 from m5.SimObject import SimObject
 
 from Device import PioDevice