pinctrl: sprd: use module_platform_driver to simplify the code

module_platform_driver() makes the code simpler by eliminating
boilerplate code.

Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Acked-by: Chunyan Zhang <zhang.lyra@gmail.com>
Link: https://lore.kernel.org/r/20200914065402.3726408-1-liushixin2@huawei.com


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