ARM: dts: imx6qdl-sabresd: Fix AR8031 phy-mode

As per kernel commit 0672d22a1924 ("ARM: dts: imx: Fix the AR803X phy-mode)
the correct phy-mode should be "rgmii-id", so fix it accordingly
to fix the Ethernet regression.

This problem has been exposed by commit:

commit 13114f38
Author: Vladimir Oltean <vladimir.oltean@nxp.com>
Date:   Thu May 7 00:11:51 2020 +0200

    phy: atheros: Explicitly disable RGMII delays

    To eliminate any doubts about the out-of-reset value of the PHY, that
    the driver previously relied on.

    If bisecting shows that this commit breaks your board you probably have
    a wrong PHY interface mode. You probably want the
    PHY_INTERFACE_MODE_RGMII_RXID or PHY_INTERFACE_MODE_RGMII_ID mode.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

Signed-off-by: Fabio Estevam <festevam@gmail.com>