imx: fec: add MAC reading from eFuses to README

Extend the documentation of the fec_mxc configuration by describing its
ability to read the ethaddr MAC address from the SoC eFuses.

Also add an example how to program the fuses for an imx5 to clarify the
byte order.

Cc: Stefano Babic <sbabic at denx.de>
Cc: Marek Vasut <marex at denx.de>
Signed-off-by: Olaf Mandel <o.mandel at menlosystems.com>