ARM: dts: am437x: fix networking on boards with ksz9031 phy

ARM: dts: am437x: fix networking on boards with ksz9031 phy

Since commit bcf3440c6dd7 ("net: phy: micrel: add phy-mode support for the
KSZ9031 PHY") the networking is broken on boards:
 am437x-gp-evm
 am437x-sk-evm
 am437x-idk-evm

All above boards have phy-mode = "rgmii" and this is worked before, because
KSZ9031 PHY started with default RGMII internal delays configuration (TX
off, RX on 1.2 ns) and MAC provided TX delay. After above commit, the
KSZ9031 PHY starts handling phy mode properly and disables RX delay, as
result networking is become broken.

Fix it by switching to phy-mode = "rgmii-rxid" to reflect previous
behavior.

Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Philippe Schenker <philippe.schenker@toradex.com>
Fixes: bcf3440c6dd7 ("net: phy: micrel: add phy-mode support for the KSZ9031 PHY")
Reviewed-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>