Fix uptime: RTC overflow is counted twice

[eflukx]

I noticed the 'DK' board implementation uptime function is broken and the uptime runs in double time. This is caused by the overflow interrupt being triggered twice. Here's a proposed patch that fixes it for me:

old handler:

#[interrupt]
fn RTC0() {
    let curr = OVERFLOWS.load(Ordering::Relaxed);
    OVERFLOWS.store(curr + 1, Ordering::Relaxed);

    // clear the EVENT register
    unsafe { core::mem::transmute::<_, RTC0>(()).events_ovrflw.reset() }
}

my replacement code (that checks whether the overflow bit is set):

#[interrupt]
fn RTC0() {
    unsafe {
        let rtc = core::mem::transmute::<_, RTC0>(());

        if rtc.events_ovrflw.read().events_ovrflw().bit_is_set() {
            OVERFLOWS.fetch_add(1, Ordering::Relaxed);
            rtc.events_ovrflw.reset()
        }
    }
}