arm64: dts: renesas: v3msk: Override EtherAVB phy-mode

As the PHY interface installed on the V3MSK board provides TX and RX
channels delays, make the "phy-mode" property a board-specific one,
meant to override the one specified in the SoC DTSI.

Follow up patches will reset the r8a77970 SoC DTSI to use "rgmii"
mode and let the board file override that.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>