pinctrl: Document pin_config_group_get() return codes like pin_config_get()

The pinconf_generic_dump_one() function makes the assumption that
pin_config_group_get() should return -EINVAL and -ENOTSUPP just like
pin_config_get() does.  Document that so it's more obvious.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>