ARM: imx: rename uart and fec device tree nodes

It has been pointed out by device tree maintainer for several times
that the generic names 'serial' and 'ethernet' should be used for
those devices per ePAPR.  Renames imx uart and fec device tree nodes
to stop them being bad examples.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>