dt-bindings: usb: add documentation for aspeed usb-vhub

Add device tree binding documentation for the Aspeed USB 2.0 Virtual HUb
Controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tao Ren <rentao.bupt@gmail.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Felipe Balbi <balbi@kernel.org>