pinctrl: add documentation for pinctrl_get_group_pins()

This patch adds a short description of how to use the newly added
pinctrl_get_group_pins function to the pinctrl documentation.

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