arm64: dts: renesas: Add compatible properties to AR8031 Ethernet PHYs

Add compatible values to Ethernet PHY subnodes representing Atheros
AR8031 PHYs on RZ/G2 boards.  This allows software to identify the PHY
model at any time, regardless of the state of the PHY reset line.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/3f1b58756f149f0c634c66abaecc88e699f4c3cc.1631174218.git.geert+renesas@glider.be