pinctrl: document the "GPIO mode" pitfall

Recently as adoption of the pinctrl framework is reaching
niches where the pins are reconfigured during system sleep
and datasheets often talk about something called "GPIO mode",
some engineers become confused by this, thinking that since
it is named "GPIO (something something)" it must be modeled
in the kernel using <linux/gpio.h>.

To clarify things, let's put in this piece of documentation,
or just start off the discussion here.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Pankaj Dev <pankaj.dev@st.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>