thermal/drivers/imx8mm: Enable ADC when enabling monitor

The i.MX 8MP has a ADC_PD bit in the TMU_TER register that controls the
operating mode of the ADC:
* 0 means normal operating mode
* 1 means power down mode

When enabling/disabling the TMU, the ADC operating mode must be set
accordingly.

i.MX 8M Mini & Nano are lacking this bit.

REFERENCE:
https://git.toradex.com/cgit/linux-toradex.git/commit/drivers/thermal/imx8mm_thermal.c?h=toradex_5.15-2.2.x-imx&id=4c12feec2dfde62dea8dabe1d2dd78ee340ba048