net: tsec: Change compatible strings to match Linux

In the case of the tsec network driver, so far there has been no
mainline user of DM_ETH where the DT bindings get used.

In the case of the mdio bus, it looks like the "fsl,tsec-mdio" string
was made up for the documentation, but there is no mainline code that
parses the "compatible" property anyway.

In both cases, there are no DT blobs that contain the old strings.

So change the documentation to "fsl,etsec2" for the Ethernet ports and
"fsl,etsec2-mdio" for the MDIO buses, which are strings that Linux also
uses, at least for LS1021A.  More compatible strings can be added once
other (PowerPC) SoCs are migrated to DM_ETH.

The current ls1021a.dtsi doesn't match what was documented for the MDIO
buses anyway (the "compatible" is "gianfar" currently). This will be
fixed in the next patch.

Fixes: 69a00875 ("doc: dt-bindings: Describe Freescale TSEC ethernet controller")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>