gpio: dt-bindings: Add GPIO on top of generic pin control

gpio: dt-bindings: Add GPIO on top of generic pin control

Traditionally, firmware will provide a GPIO interface or a pin control
interface.  However, the SCMI protocol provides a generic pin control
interface and the GPIO support is built on top of that using the normal
pin control interfaces.  Potentially, other firmware will adopt a
similar generic approach in the future.

Document how to configure the GPIO device.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>