ARM: dts: renesas: Add compatible properties to KSZ9031 Ethernet PHYs

Add compatible values to Ethernet PHY subnodes representing Micrel
KSZ9031 PHYs on RZ/G1 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>
Tested-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/ce8ae6b199fa244315a008ae31891a808ca1948d.1631174218.git.geert+renesas@glider.be