doc: dt-bindings: tsec: Correct the Ethernet port compatible string

Change the compatible string to "fsl,etsec2" for the Ethernet ports,
which is used in the current driver's match table.

Fixes: 69a00875 ("doc: dt-bindings: Describe Freescale TSEC ethernet controller")
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Acked-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>