soc: mediatek: PMIC wrap: move wdt_src into the pmic_wrapper_type struct

Different SoCs will use different bitmask for the wdt_src. This patch
defines the bitmask in the pmic_wrapper_type struct. This allows us to
support new SoCs with a different bitmask to the one currently used.

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