yang: add operational data to frr-interface model

yang: add operational data to frr-interface model

module: frr-interface
  +--rw lib
     +--rw interface* [name vrf]
        +--rw name           string
        +--rw vrf            frr-vrf:vrf-ref
        +--rw description?   string
        +--ro state
           +--ro if-index?      int32
           +--ro mtu?           uint16
           +--ro mtu6?          uint32
           +--ro speed?         uint32
           +--ro metric?        uint32
           +--ro flags?         if-flags-type
           +--ro type?          identityref
           +--ro phy-address?   ietf-yang-types:mac-address

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>