pinctrl: sprd: Allow the SPRD pinctrl driver building into a module

Change the config to 'tristate' and export some symbols needed by modules
to allow the Spreadtrum pinctrl driver building into a module.

Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Link: https://lore.kernel.org/r/d7239f3c7379e402f665fc8927f635ac56691380.1582776447.git.baolin.wang7@gmail.com


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