config: Use null memory for DRAM sweep script
config: Use null memory for DRAM sweep script

Do not waste time when we do not care about the data.
(transplanted from a4a2ba97a6549a976ab5954f5aea77f70d0b74e3)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin