ARM: dts: imx6sx-udoo-neo: Properly configure ENET_REF

By default, the ENET_REF is configured at 125MHz on i.MX6SX, which works
well for boards that operate in RGMII mode.

The imx6sx-udoo-neo has a KSZ8091 Ethernet PHY that is connected via
RMII interface, so a 50MHz ENET_REF clock is expected.

Describe the IMX6SX_CLK_ENET_REF accordingly.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>