ARM: imx6: Fetch MAC address in board_init_late() on DH iMX6 PDK2

This is needed to obtain the MAC from EEPROM/OTP only after the final
env is populated, otherwise the ethaddr might be overriden.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Claudius Heine <ch@denx.de>
Cc: Harald Seiler <hws@denx.de>
Cc: Ludwig Zenz <lzenz@dh-electronics.com>
Cc: Stefano Babic <sbabic@denx.de>