memory: mtk-smi: Rename smi_gen to smi_type

memory: mtk-smi: Rename smi_gen to smi_type

Prepare for adding smi sub common. Only rename from smi_gen to smi_type.
No functional change.

About the current "smi_gen", we have gen1/gen2 that stand for the
generation number for HW. I plan to add a new type(sub_common), then the
name "gen" is not proper.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Ikjoon Jang <ikjn@chromium.org>
Link: https://lore.kernel.org/r/20210914113703.31466-5-yong.wu@mediatek.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>