ARM: dts: imx6ul-prti6g: configure ethernet reference clock parent

On this board the PHY is the ref clock provider. So, configure ethernet
reference clock as input.

Without this patch we have relatively high amount of dropped packets.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>