cpu/samd5x: don't run DFLL on-demand
cpu/samd5x: don't run DFLL on-demand

The DFLL on samd5x has a hardware bug that requires a special
re-enabling sequence when it is disabled and then re-enabled again.

When running the clock on-demand, the hardware handles the disabling
and re-enabling so that sequence does not get executed.

To reproduce, run `tests/periph_uart` on `same54-xpro`.

Without this patch the test will get seemingly stuck on `sleep_test()`.
(In fact it keeps running, but the DFLL has the wrong frequency so the
UART baudrate is wrong).

In this test, on `same54-xpro` only UART0 is sourced from DFLL.
So if the UART is disabled the DFLL will be turned off as well.

OlegHahm/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,yogo1212/RIOT,basilfx/RIOT,OlegHahm/RIOT,yogo1212/RIOT,basilfx/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,yogo1212/RIOT,ant9000/RIOT,OTAkeys/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,yogo1212/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,basilfx/RIOT,yogo1212/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,basilfx/RIOT,miri64/RIOT,jasonatran/RIOT,authmillenon/RIOT,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,kYc0o/RIOT,kaspar030/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT