pinctrl: pinctrl-aspeed-g6: Add more settings for USB2AHP function

AST2600 USB2AHP (USB PortA: PCIe EHCI to PHY) function needs to set the
register SCUC20[16]. Set it to enable the PCIe EHCI device on PCIe bus.
Besides, also add USB2AHP signal expressions into pin declarations.

Signed-off-by: Joe Wang <joe_wang@aspeedtech.com>
Link: https://lore.kernel.org/r/20230920103332.274151-1-joe_wang@aspeedtech.com


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