Hi,
To use the internal RTC of BD71847 with the IMX8M-MINI controller, you will need to interface the two using the I2C protocol. The BD71847 power management IC has an inbuilt RTC which can be accessed via I2C.
Assuming you have already set up the I2C communication between the IMX8M-MINI and the BD71847, you can access the RTC registers of the BD71847 to set and get the time.
Regards