Documentation/gpio.txt: Explain expected pinctrl interaction

Update gpio.txt based on recent discussions regarding interaction with the
pinctrl subsystem.

Previously, gpio_request() was described as explicitly not performing any
required mux setup operations etc.

Now, gpio_request() is explicitly as explicitly performing any required mux
setup operations where possible. In the case it isn't, platform code is
required to have set up any required muxing or other configuration prior to
gpio_request() being called, in order to maintain the same semantics.

This is achieved by gpiolib drivers calling e.g. pinctrl_request_gpio() in
their .request() operation.

Cc: Randy Dunlap <[email protected]>
Cc: [email protected]
Signed-off-by: Stephen Warren <[email protected]>
Reviewed-by: Linus Walleij <[email protected]>
Signed-off-by: Grant Likely <[email protected]>

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