net/fsl_pq_mdio: Document supported compatibles

The device tree binding(s) document has fallen out of sync with the
driver code. Update the list of supported devices to reflect current
driver capabilities

Change-Id: I440d8de2ee2d9c3b7b23e69b3da851cab18a4c9a
Signed-off-by: Shruti Kanetkar <Kanetkar.Shruti@gmail.com>
Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>