riscv: sifive_test: Add reset functionality

This adds a reset opcode for sifive_test device to trigger a system
reset for testing purpose.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>