verdin-imx8mm: fix ethernet

verdin-imx8mm: fix ethernet

Turns out Microship (formerly Micrel) meanwhile integrated proper
support for the DLL setup on their KSZ9131. Unfortunately, this
conflicts with our previous board code doing that.
Fix this by getting rid of our board code and just relying on the
generic implementation relying on rgmii-id being used as phy-mode.

Fixes: commit c6df0e2ffdc4
       ("net: phy: micrel: add support for DLL setup on ksz9131")
Fixes: commit af2d3c91d877
       ("ARM: dts: imx8mm-verdin: Set PHY mode to RGMII-ID")
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>