soc: mediatek: PMIC wrap: add a slave specific struct

This patch adds a new struct pwrap_slv_type that we use to store the slave
specific data. The patch adds 2 new helper functions to access the dew
registers. The slave type is looked up via the wrappers child node.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>