ARM: dts: omap5-uevm: Allow bootloader to configure USB Ethernet MAC

Note that with 9730 the wiring is different compared to 9514 found on
beagleboard xm for example.

On beagleboard xm we have:

/sys/bus/usb/devices/1-2	hub
/sys/bus/usb/devices/1-2.1	9514

While on omap5-uevm we have:

/sys/bus/usb/devices/1-2	hub
/sys/bus/usb/devices/1-3	9730

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