dt-bindings: pinctrl: aspeed: Add g5 USB functions

The Aspeed AST2500 SoC contains a number of USB controllers:

* USB 1.1 Host Controller
* USB 2.0 Host Controller (x2)
* USB 2.0 Virtual Hub
* USB 2.0 Device Controller
* USB 1.1 HID Controller

The controllers are exposed via two USB ports with functionality muxed
as required. The following table illustrates the relationships between
the ports and the controllers via the mux function names:

Port  | USB Version  | USB Mode     | Mux Function
------|--------------|--------------|-------------
A     | 2.0          | Virtual Hub  | USB2AD
A     | 2.0          | Host         | USB2AH
B     | 1.1          | HID          | USB11BHID
B     | 2.0          | Device       | USB2BD
B     | 2.0          | Host         | USB2BH

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>