ARM: dts: imx6: Fix Ethernet PHY mode on Ventana boards

Gateworks Ventana boards seem to need "RGMII-ID" (internal delay)
PHY mode, instead of simple "RGMII", for their Marvell 88E1510
transceiver. Otherwise, the Ethernet MAC doesn't work with Marvell PHY
driver (TX doesn't seem to work correctly).

Tested on GW5400 rev. C.

This bug affects ARM Fedora 23.

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Acked-by: Tim Harvey <tharvey@gateworks.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>