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.
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