mcause/scause values when H is implemented

[pdonahue-ventana]

The mcause and scause CSR descriptions have tables of legal values that include software check (18), hardware error (19), and (for mcause only) double trap (16). The hedeleg description mentions these values but the "Machine and supervisor cause register (mcause and scause) values when the hypervisor extension is implemented" table lists values 16-19 as reserved. Software check and hardware error should be listed. I haven't read double trap since it was an early draft but I think that it's a legal value for scause if H is implemented. It's certainly a legal value for mcause.