pinctrl: digicolor: add missing platform_set_drvdata() call

gc_pinctrl_remove() calls platform_get_drvdata(), but I see neither
platform_set_drvdata() nor dev_set_drvdata() anywhere in this driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>