arm64: dts: renesas: r8a77970: Set EtherAVB phy mode to "rgmii"

Set the "phy-mode" property of EtherAVB device to "rgmii" and let board
files override it if the installed PHY layer provides delays for the
RX/TX channels.

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