ARM: dts: imx8mm-verdin: drop rgmii_rxc_dly/txc_dly

The FEC in the i.MX8MM doesn't support this feature. So don't pretend one
can use it.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Acked-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>