pinctrl: core: Make pinctrl_init_controller() static

pinctrl_init_controller() is not used outside core.c, thus make it
static and prevent compiler to warn.

drivers/pinctrl/core.c:1943:21: warning: no previous prototype for ‘pinctrl_init_controller’ [-Wmissing-prototypes]
 struct pinctrl_dev *pinctrl_init_controller(struct pinctrl_desc *pctldesc,
                     ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>