rdma: Add json output to link object

An example for the JSON output for two devices system.

root@mtr-leonro:~# rdma link -d -p -j
[{
        "ifindex": 1,
        "port": 1,
        "ifname": "mlx5_0/1",
        "subnet_prefix": "fe80:0000:0000:0000",
        "lid": 13399,
        "sm_lid": 49151,
        "lmc": 0,
        "state": "ACTIVE",
        "physical_state": "LINK_UP",
        "caps": ["AUTO_MIG"
        ]
    },{
        "ifindex": 2,
        "port": 1,
        "ifname": "mlx5_1/1",
        "subnet_prefix": "fe80:0000:0000:0000",
        "lid": 13400,
        "sm_lid": 49151,
        "lmc": 0,
        "state": "ACTIVE",
        "physical_state": "LINK_UP",
        "caps": ["AUTO_MIG"
        ]
    }
]

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>