The MX8M Mini does not have an internal 32k ring oscillator, and does not support the use of a crystal.
Please see the datasheet, Section 3.1.4:
The RTC_XTALI is used for low-frequency functions. It supplies the clock for wake-up circuit, power-down real time clock operation, and slow system and watch-dog counters. The clock input can only be connected to an external oscillator. RTC_XTALO should be directly connected to VDD_SNVS_0P8.
and Section 3.4.2:
An external 32.768 kHz oscillator is necessary.
Regards