slave_i2c_device: Inform about GD32 MCU and whether user regulator support is comiled in or not in GET_STATUS_WORD
Add new status word bit GD32_MCU_STSBIT
and set it to 1 in GD32 firmware.
Add new status word bit USER_REGULATOR_NOT_SUPPORTED_STSBIT
and set it to 1 if user regulator support is not compiled in.
These changes are backwards compatible with STM32 firmware.