ARM: dts: imx7d-pinfunc: add input mux for ENET2 mdio

Add the missing input mux for ENET2 mdio. Without this setting, it is not
possible to read the MDIO answers back from the PHY.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>