soc: mediatek: PMIC wrap: split SoC specific init into callback

This patch moves the SoC specific wrapper init code into separate callback
to avoid pwrap_init() getting too large. This is done by adding a new
element called init_special to pmic_wrapper_type. Each currently supported
SoC gets its own version of the callback and we copy the code that was
previously inside pwrap_init() to these new callbacks. Finally we point the
2 instances of pmic_wrapper_type at the 2 new functions.

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