ARM: fdt support: Add usbethaddr as an acceptable MAC

A board that has a USB ethernet device only may set the usbetheraddr
and not the ethaddr.
ethaddr will be the default MAC address that is chosen and if that
is not populated then the usbethaddr is looked at.  If neither are set
then then device tree blob is not modified.

Signed-off-by: Dan Murphy <dmurphy@ti.com>