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.

Ported from Linux kernel commit 1c65ef1c71e473c00f2a7a1b9c140f0b4862f282 .

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
---
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Michal Simek <michal.simek@amd.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Ramon Fried <rfried.dev@gmail.com>