config: Change mem_range attribute naming in ARM SimpleSystem

config: Change mem_range attribute naming in ARM SimpleSystem

MemConfig.config() expects memory ranges to be defined in a particular
way. This patch changes the naming of the mem_range attribute in
SympleSystem to enable use of MemConfig for configuring the memory.

Change-Id: I4964c136e53a99c69ff5e086cacb929aa435168d
Signed-off-by: Gabor Dozsa <gabor.dozsa@arm.com>
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4200
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>