Missing SATP checks.

[pawks]

Even though the mode field is warl, the spec lists various constraints on the allowed behavior of the various fields in satp register. These checks aren't implemented right now.

• Behaviour of writes when satp.mode is written with an illegal value. The spec says the following:

    if satp is written with an unsupported MODE, the entire write has no effect; 
    no fields in satp are modified.
  

  Hence all the fields in satp will need to contain satp.mode as a dependency and the only allowed illegal behavior on illegal mode fields is unchanged.