ARM: dts: imx6qp-prtwd3: configure ENET_REF clock to 125MHz

By default ENET_REF is configured to 50MHz, which is usable for the RMII
link. In case RGMII is used, we need 125MHz clock.

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