soc: mediatek: PMIC wrap: SPI_WRITE needs a different bitmask for MT2701/7623

Different SoCs will use different bitmask for the SPI_WRITE command. 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>