mx28: Let imx_get_mac_from_fuse be common for mx28

Let imx_get_mac_from_fuse function be a common function, so that other
mx28 boards can reuse it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>