dts: powerpc: p2020rdb: Add eTSEC DT nodes

P2020RDB implements 3 enhanced three-speed Ethernet controllers,
and the connection is shown below:
    eTSEC1: Connected to RGMII switch VSC7385
    eTSEC2: Connected to SGMII PHY VSC8221
    eTSEC3: Connected to SGMII PHY AR8021

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>