pinctrl: update GPIO range doc

This updates the GPIO range documentation with the API changes
for sparse/random/arbitrary pin-to-GPIO mappings.

Reviewed-by: Christian Ruppert <christian.ruppert@abilis.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>