imx: refactor imx_get_mac_from_fuse

Move imx_get_mac_from_fuse to a new mac.c for i.MX6/7.
Since fuse regs structure are different for i.MX6/7, use mac
address offset in code and define a new local struture
imx_mac_fuse.

Also sort the config order.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>