memory: mtk-smi: Use a struct for the platform data for smi-common

Use a struct as the platform special data instead of the enumeration.

Also there is a minor change that moving the position of
"enum mtk_smi_gen" definition, this is because we expect define
"struct mtk_smi_common_plat" before it is referred.

This is a preparing patch for mt8183.

Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>