memory: mtk-smi: Allow building as module

Add support for building the SMI driver as module. Switch MTK_SMI to
tristate, and add module_exit/module_license.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Link: https://lore.kernel.org/r/20210126060055.11050-1-yong.wu@mediatek.com


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>