UPSTREAM: 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>
(cherry picked from commit 50fc8d92)

BUG=b:174513569

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Change-Id: I6d4c9e14304d6d42e4a1e8ef441ae8591495b2df