ARM: mxs: read correct values when setting up MAC

Currently, the MAC address from the second ethernet is generated from the
crypto-key (and not a customer reg) because of a wrong index to the ocotp
array.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>