python: fix "fatal" usage in fdthelper

"fatal" was not correctly imported in the fdthelper module,
which caused a crash when reporting errors.

Change-Id: I7ee9dcde1f0288e11e56dba67ead4aa2d6d67e02
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23753
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/util/fdthelper.py

@@ -43,6 +43,7 @@ from m5.ext.pyfdt import pyfdt
 import re
 import os
 from m5.SimObject import SimObject
+from m5.util import fatal
 
 class FdtProperty(pyfdt.FdtProperty):
     """Create a property without values."""