ARM: dts: imx6sx-sabreauto: Fix the phy-mode on fec2

Commit 0672d22a ("ARM: dts: imx: Fix the AR803X phy-mode") fixed the
phy-mode for fec1, but missed to fix it for the fec2 node.

Fix fec2 to also use "rgmii-id" as the phy-mode.

Cc: <stable@vger.kernel.org>
Fixes: 0672d22a

 ("ARM: dts: imx: Fix the AR803X phy-mode")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>