The INTFLAGS register is cleared by writing a 1 to the corresponding interrupt
flag bit. From the samr21's manual:
> Writing a zero to this bit has no effect.
> Writing a one to this bit clears the Compare 0 interrupt flag.
This is a common pattern in flag registers.
This RTT driver is using or-equal to clear the flags, which means it can
possibly clear other interrupts. There's a small chance that one event is
missed if it happens very close to another event.
Credits to @benpicco, @dylad for pointing out missing fixes.
The currently supported SAM0 MCUs (samd21, saml21, saml1x) share the
same RTC peripheral, yet each of them carries it's own copy of the RTT
driver.
Unify the drivers and move them to sam0_common.
