pinctrl: remove mutex lock in groups show

Mutex is locked duplicatly by pinconf_groups_show() and
pin_config_group_get(). It results dead lock. So avoid to lock mutex
in pinconf_groups_show().

Cc: stable@kernel.org
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>