soc: mediatek: pwrap: add common way for setup CS timing extenstion

Multiple platforms would always use their own way handling CS timing
extension on the bus which leads to a little bit code duplication.
Therefore, the patch groups the similar logic to handle CS timing
extension into the common function which allows the following SoCs
have more reusability for configing CS timing.

Signed-off-by: Chenglin Xu <chenglin.xu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>