arch-arm, configs: Create single instance of DTB autogeneration

arch-arm, configs: Create single instance of DTB autogeneration

This patch is rewriting the DTB autogeneration functions available in
fs_bigLITTLE.py and fs.py as a single method in the GenericArmSystem
so that other configuration scripts can make use of it.

Change-Id: I492bbf77e6b0ac5c5fbdbc75c0eecba29bd63bda
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15958
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>