Dpdk Backend: Register extern support for bfrt.json

[usha1830]

This PR adds support for register extern in bfrt.json.
A new table is emitted for the register instance. This is as expected by the p4-dpdk-target.

{
  "name" : "ip.MyIC.reg",
  "id" : 369588800,
  "table_type" : "Register",
  "size" : 1024,
  "annotations" : [],
  "depends_on" : [],
  "key" : [
    {
      "id" : 65557,
      "name" : "$REGISTER_INDEX",
      "repeated" : false,
      "annotations" : [],
      "mandatory" : true,
      "match_type" : "Exact",
      "type" : {
        "type" : "uint32"
      }
    }
  ],
  "data" : [
    {
      "mandatory" : false,
      "read_only" : false,
      "singleton" : {
        "id" : 65557,
        "name" : "$REGISTER_INDEX",
        "repeated" : false,
        "annotations" : [],
        "type" : {
          "type" : "bytes",
          "width" : 16
        }
      }
    }
  ],
  "supported_operations" : ["Sync"],
  "attributes" : []
}

[usha1830]

These changes are consumed by p4-dpdk-target and are verified as mentioned in this comment
#3711 (comment)

[hanw]

LGTM