pinctrl: sprd: Restrict PINCTRL_SPRD to ARCH_SPRD or COMPILE_TEST

The Spreadtrum pinctrl drivers are only useful when building for a
Spreadtrum platform.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>