add associations schema

[drom]

associations: {
  type: 'object',
  properties: {
    clock: {type: 'string'}, // clock busInterface name
    reset: {type: 'string'}  // reset busInterface name
  }
},

here: https://github.com/sifive/duh-schema/blob/master/lib/busInterface.js#L22

[mwachs5]

can this be applied to individual wires/ports, or only for full bus types?

[drom]

You can bundle one or more ports and add assotiations.