SAMD51: change of DAC power setting

[digixx]

Fixes wrong DAC output levels at high data rate
closes #3796

[jepler]

We got the present setting when I tried copying Arduino register settings to CircuitPython: 61698eb

The mismatch between the associated GCLK and the register setting was noted in that commit.

Before that, it appears we used GCLK5 at 2MHz and used DAC_DACCTRL_CCTRL_CC1M which was "correct" (1M is for 1.2MHz < GCLK_DAC ≤ 6MHz)

I don't know whether I made an error selecting the combination of CC100K and 12MHz, or whether that is truly what Arduino does--a quick grep in Adafruit's ArduinoCore-samd didn't give me the answer.

This seems "correct" and the submitter has tested it, so let's give this a try. We'll have at least one beta to check it out in.

Testing performed: none

[jepler]

Thank you @digixx .