soc: mediatek: pwrap: add pwrap_read32 for reading in 32-bit mode

Some regulators such as MediaTek MT6380 has to be read in 32-bit mode.
So the patch adds pwrap_read32, rename old pwrap_read into pwrap_read16
and one 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>