dt-bindings: pinctrl: aspeed-g6: Add USB functions and groups

The AST2600 provides two USB ports (A and B) that expose various host,
device and HID functions.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20191202061432.3996-2-andrew@aj.id.au


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>