dm: pinctrl: Add a way for a GPIO driver to obtain a pin function

GPIO drivers want to be able to show if a pin is enabled for input, output,
or is being used by another function. Some drivers can easily find this
and the code is included in the driver. For some SoCs this is more complex.
Conceptually this should be handled by pinctrl rather than GPIO. Most
pinctrl drivers will have this feature anyway.

Add a method by which a GPIO driver can obtain the pin mux value given a
GPIO reference. This avoids repeating the code in two places.

Signed-off-by: Simon Glass <sjg@chromium.org>