dt-bindings: usb: document hub and host-controller properties

Hub nodes and host-controller nodes with child nodes must specify values
for #address-cells (1) and #size-cells (0).

Also make the definition of the related reg property a bit more
stringent, and add comments to the example source.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>