pinctrl: generic: Make 'conf_items' static

'conf_items' is used only in this file. Silences the below
sparse warning:
drivers/pinctrl/pinconf-generic.c:37:24: warning:
symbol 'conf_items' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>