According to the R-Car Gen3 Hardware Manual Errata for Rev 1.00 of
August 24, 2018, the TX clock internal delay mode doesn't support
on R-Car E3. This patch fixes EthernetAVB phy mode to rgmii.
This is achieved by simply dropping the phy-mode property from
r8a77990-ebisu.dts as the default property for this for r8a77990,
as set in r8a77990.dtsi, is "rgmii".