arm64: dts: renesas: r8a77990: ebisu: Fix EthernetAVB phy mode to rgmii

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".

Signed-off-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@renesas.com>
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>