CVA6-DV : Update CVA6-DV

Created by: AyoubJalali

Hello, So this update of the CVA6-DV is about the custom instruction use in cvxif, so we add the cutom instruction definition in the cva6_instr_pkg, instead of copy them to the riscv_instr_pkg then use it. it's just an update to be more independent from the RISCV-DV