ARM: dts: pandaboard: Allow bootloader to configure USB Ethernet MAC

Inspired by a patch for beagleboard xm by Laurent Pinchart
<laurent.pinchart@ideasonboard.com>, similar patch also works for
pandaboard. The only difference is that the hub is address 1 instead
of 2.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>