qemu_cortex_m0: Disable QEMU icount feature

The NRF timer emulated by QEMU is not behaving correctly when icount is
used.

This is easily reproducible with the following snippet:

  unsigned int key = irq_lock();
  k_busy_wait(15000);
  irq_unlock(key);
  printk("done\n");

where the `printk` is never executed and `k_busy_wait` is never left.

This particular code is only mis-behaving when `-icount` is used:

This works fine:

qemu-system-arm -cpu cortex-m0 -machine microbit -nographic
    -kernel build/zephyr/zephyr.elf -rtc clock=vm

This doesn't work:

qemu-system-arm -cpu cortex-m0 -machine microbit -nographic
    -kernel build/zephyr/zephyr.elf -rtc clock=vm
    -icount shift=6,align=off,sleep=off

Signed-off-by: Carlo Caione <ccaione@baylibre.com>