When changing the clock configuration while the RTC is running, the
RTC may end up in an undefined state that leaves it unresponsive.
The RTC is not reset to stay persistent across reboots/hibernate, so
it will not be reset on init.
Instead, disable the RTC while configuring the clocks, rtc_init() will
take care of re-enabling it.
@dylad introduced this workaround for saml21, samd5x needs it too.
To reproduce, set the CLOCK_CORECLOCK of a samd5x board (e.g. same54-xpro)
to 48 MHz.
Run any RTC application. The CPU will be stuck in _wait_syncbusy() after
a reboot.
This patch will fix this. (You will need to power-cycle the board if the
RTC has entered the stuck state as it will never be reset.)
d12abe6a2b