net: freescale: fec: add support for optional enet_out clk

Some MX28 boards need the internal enet_out clock to be enabled. So, do
this in the driver iff the clock was referenced via devicetree.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: David S. Miller <davem@davemloft.net>