OMAP3: beagle-xm: generate fake USB ethernet MAC address from dieid

Similar to OMAP5uEVM, PandaBoard, BeagleBoard-XM has a USB based
ethernet without MAC address embedded. So fake a MAC address following
the similar strategy used on OMAP5 and PandaBoard family.

Signed-off-by: Nishanth Menon <nm@ti.com>