Exception priority table should be consistent with latest Priv Spec

[jhauser-us]

In Section 5.3, "Priority", the Debug Spec's table of exception priorities should be made consistent with the corresponding tables in the latest Privileged Spec draft, found in that document in Section 3.1.15, "Machine Cause Register (mcause)", and (for the hypervisor extension) Section 9.6.1, "Trap Cause Codes".