dt-bindings: usb: usb251xb: support usage case without I2C control

dt-bindings: usb: usb251xb: support usage case without I2C control

Currently, the usb251xb assumes i2c control, but from HW point of
view, the hub supports usage case without any i2c control, I.E we
only want the gpio controls, for example the following dt node:

usb-hub {
	compatible = "microchip,usb2512b";
	reset-gpios = <&porta 8 GPIO_ACTIVE_LOW>;
};

Modify the dt-binding of usb2512b to support this usage case, and add
the usage example to the examples section.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250825234509.1041-2-jszhang@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>