Include enums to the P4InfoConstants.java file

[Yi-Tseng]

Now we can use the serializale_enums from the p4info file to generate constants, here is anexample of serializable_enums:

type_info {
  serializable_enums {
    key: "BridgedMdType_t"
    value {
      underlying_type {
        bitwidth: 8
      }
      members {
        name: "INVALID"
        value: "\000"
      }
      members {
        name: "INGRESS_TO_EGRESS"
        value: "\001"
      }
      members {
        name: "EGRESS_MIRROR"
        value: "\002"
      }
      members {
        name: "INGRESS_MIRROR"
        value: "\003"
      }
    }
  }
}