pinctrl: document semantics vs GPIO

The semantics of the interactions between GPIO and pinctrl may be
unclear, e.g. which one do you request first? This amends the
documentation to make this clear.

Reported-by: Domenico Andreoli <cavokz@gmail.com>
Acked-by: Domenico Andreoli <domenico.andreoli@linux.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>