soc: mediatek: PMIC wrap: add wrapper callbacks for init_reg_clock

Split init_reg_clock up into SoC specific callbacks. The patch also
reorders the code to avoid the need for callback function prototypes.

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