ARM: dts: ls1021: Fix SGMII PCS link remaining down after PHY disconnect

Each eTSEC MAC has its own TBI (SGMII) PCS and private MDIO bus.
But due to a DTS oversight, both SGMII-compatible MACs of the LS1021 SoC
are pointing towards the same internal PCS. Therefore nobody is
controlling the internal PCS of eTSEC0.

Upon initial ndo_open, the SGMII link is ok by virtue of U-boot
initialization. But upon an ifdown/ifup sequence, the code path from
ndo_open -> init_phy -> gfar_configure_serdes does not get executed for
the PCS of eTSEC0 (and is executed twice for MAC eTSEC1). So the SGMII
link remains down for eTSEC0. On the LS1021A-TWR board, to signal this
failure condition, the PHY driver keeps printing
'803x_aneg_done: SGMII link is not ok'.

Also, it changes compatible of mdio0 to "fsl,etsec2-mdio" to match
mdio1 device.

Fixes: 055223d4 ("ARM: dts: ls1021a: Enable the eTSEC ports on QDS and TWR")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>