tests: Reduce checkpoint interval used by realview regressions

tests: Reduce checkpoint interval used by realview regressions

With the current interval, the realview(64)-simple-atomic-checkpoint
regressions are not actually checkpointing since they finish boot
(and hit m5 exit) before creating the first checkpoint.

Change-Id: I297864ccb7ec8a818c9eccd94406b69d89d1f8d3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Richard Cooper <richard.cooper@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27709
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>