arm64: dts: renesas: Add compatible properties to KSZ9031 Ethernet PHYs

Add compatible values to Ethernet PHY subnodes representing Micrel
KSZ9031 PHYs on R-Car Gen3 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>
Link: https://lore.kernel.org/r/07bd7e04dda9e84cde0664980f0b1a6d69e03109.1631174218.git.geert+renesas@glider.be