arm: dts: imx53-cx9020: fix packetloss on fec_mxc

The pinmuxing for i.MX53 FEC ethernet copied from
<kernel>/arch/arm/boot/dts/imx53-qsb-common.dtsi (at least until v4.9)
was bad. It is different from the manual pinmuxing in
<u-boot>/board/freescale/mx53loco/mx53loco.c which was used in
cx9020 implementation previously before mainlining into u-boot.
It seems the bug in imx53-qsb kernel device tree is hidden for so long,
because it was never used, by the kernel driver.

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>