Implement the AST2400 USB functions as described by the devicetree
bindings. Three ports are fully documented in the datasheet and exposed
through the bindings and pinctrl, though there are remnants of
documentation for a fourth port muxed with GPIO pins GPIOQ6 and GPIOQ7.
The implementation is updated to reflect this but the function and
group are not exposed.

Disregarding the mostly undocumented fourth port, the USB functions are
an outlier with respect to the rest of the muxed functionality on the
AST2400 as GPIO is not supported on these pins.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>