ARM: mxs: store mac address read from OTP in device tree

The non-DT boot reads the mac from OTP and pass it to fec driver via
platform data.  The patch provides an equivalent support for device
tree boot, with reading mac from OTP and store it in device tree,
and fec driver can get the mac from device tree at its probe time.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>