arm64: dts: renesas: salvator-common: Override EtherAVB phy-mode

As the PHY interface installed on the Salvator-X[S] board, provides TX
channel delay, 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 r8a7795/96/965 SoC DTSI to use "rgmii"
mode and let the board files override that.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>