dbg: ARM: dts: boneblack: add USB topology and serdev nodes

Add a hub device and two USB devices, of which one has a combined node.

Note that we need to represent the serial ports as well -- consider
devices with multiple ports per interface; which one should serdev
use? Sibling devices can also be described this way (e.g. gpio@0), and
would need to use the same address size.

Also note that serial ports have a standardised node name in ePAPR.
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