@haydin, @bhuvanchandra.dv, mac address assignment is Ethernet device specific, hence all depends what kind of SPI based ethernet device you plan to use…
The FEC driver, which is used for the on-board ethernet controller, is able to read the MAC address from the device tree as well as adopt the value from the FEC registers already set by the boot loader (for the exact details examine the fec_get_mac function in drivers/net/ethernet/freescale/fec_main.c).
Toradex assigns two MAC addresses from our OUI 00:14:2d (Organizationally Unique Identifier). Those two MAC addresses are ment to be assigned for the two available Ethernet controller, however I don’t see any problem to use it for another Ethernet device.
Those two MAC addresses are generated from the serial number and are available as environment variables in U-Boot ( ethaddr and eth1addr). U-Boot assigns this environment variables to the Ethernet device(s) specified in the device tree aliases section (our dual-eth device tree has this section specified).
For the SPI device: I would make sure the device driver is able to retrieve the MAC address from the device tree. If this is the case, an alias entry which links to your second Ethernet device (the SPI slave node) should be enough to assign our second MAC address to your device.