pinctrl: sink pinctrldev_list_mutex

The pinctrldev_list_mutex is sinked into the functions that
actually traverse the list and lock it there. The code makes
much more sense in this way. All the callers are in
non-performance critical paths and the code is way more
readable this way.

Also refactor the function get_pinctrl_dev_from_devname() to
follow the design pattern of get_pinctrl_dev_from_of_node()
which is slightly simpler.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>