ARM: dts: mxs: add enet_out clock to devicetree

Put the clock to the devicetree, so the driver can take care of it
later. Then, we don't have to do the enabling as a workaround in board
init.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
[shawn.guo: add enet_out into imx28.dtsi and overwrite it for m28evk]
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>