ARM: dts: imx: Fix the AR803X phy-mode

Commit 6d4cd041 ("net: phy: at803x: disable delay only for RGMII mode")
exposed an issue on imx DTS files using AR8031/AR8035 PHYs.

The end result is that the boards can no longer obtain an IP address
via UDHCP, for example.

Quoting Andrew Lunn:

"The problem here is, all the DTs were broken since day 0. However,
because the PHY driver was also broken, nobody noticed and it
worked. Now that the PHY driver has been fixed, all the bugs in the
DTs now become an issue"

To fix this problem, the phy-mode property needs to be "rgmii-id",  which
has the following meaning as per
Documentation/devicetree/bindings/net/ethernet.txt:

"RGMII with internal RX and TX delays provided by the PHY, the MAC should
not add the RX or TX delays in this case)"

Tested on imx6-sabresd, imx6sx-sdb and imx7d-pico boards with
successfully restored networking.

Based on the initial submission from Steve Twiss for the
imx6qdl-sabresd.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Soeren Moch <smoch@web.de>
Tested-by: Steve Twiss <stwiss.opensource@diasemi.com>
Tested-by: Adam Thomson <Adam.Thomson@diasemi.com>
Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Tested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>