Fix comment on pinctrl_gpio_range.pin_base

The comment introduced with the recently added pinctrl_gpio_range.pins
element was wrong. This corrects it.
Thanks to Patrice Chotard for pointing this out.

Signed-off-by: Christian Ruppert <christian.ruppert@abilis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>