Implement the AST2500 USB functions as described by the devicetree
bindings. The AST2500 exposes five USB controllers through two USB
ports. Similar to the AST2400, the pins exposing USB are outliers with
respect to the rest of the pinmux as they not capable of GPIO.
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>