net: tsec: Add the compatible string "gianfar" support

Add compatible string "gianfar" support and update the
device-tree-bindings doc.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>