ARM: dts: renesas: Add compatible properties to LAN8710A Ethernet PHYs

Add compatible values to Ethernet PHY subnodes representing SMSC
LAN8710A PHYs on RZ/A1 and R-Mobile A1 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/247dc2074dae149af07b6d014985ad30eb362eda.1631174218.git.geert+renesas@glider.be