pinctrl.txt: Remove outdated information

Returning -EAGAIN is no longer supported by pin_config_group_set()
since ad42fc6c ("pinctrl: rip out the direct pinconf API")

Remove the relevant section from the documentation.

Signed-off-by: Ramon Fried <ramon.fried@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>