Currently, store instructions are assumed to have two source registers.
However, since we are supporting the RISC-V CMO instructions, which
are Store instructions in gem5 but they only have one source register.
This change allows printing disassembly of Store instructions with
one source register.
Change-Id: I4dd7818c9ac8a89d5e10e77db72248942a25e938
Signed-off-by: Hoa Nguyen <hn@hnpl.org>