configs, arm: Fix incorrect use of mem_range in bL example

configs, arm: Fix incorrect use of mem_range in bL example

The change "config: Change mem_range attribute naming in ARM
SimpleSystem" modified the SimpleSystem class to be compatible with
the MemConfig utility script. While doing so, the way we report the
memory ranges supported by the system changed, which broke the bL
example configration. This changeset introduces the necessary changes
to make the script work again.

Change-Id: I789987950ff04b6c5ae1c8b807355bcba34f6b3c
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4380
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>