pinctrl: protect pinctrl_list add

We have few fedora bug reports about list corruption on pinctrl,
for example:
https://bugzilla.redhat.com/show_bug.cgi?id=1051918



Most likely corruption happen due lack of protection of pinctrl_list
when adding new nodes to it. Patch corrects that.

Fixes: 42fed7ba ("pinctrl: move subsystem mutex to pinctrl_dev struct")
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>