soc: mediatek: pwrap: add pwrap_write32 for writing in 32-bit mode

Some regulators such as MediaTek MT6380 also has to be written in
32-bit mode. So the patch adds pwrap_write32, rename old pwrap_write
into pwrap_write16 and one additional function pointer is introduced
for increasing flexibility allowing the determination which mode is
used by the pwrap slave detection through device tree.

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