drivers: timer: stm32 lptim fix long time interrupt locking

The waiting of the flag ARROK could be quite long (100µs-200µs in my
test). During this time, the interrupts are locked that is not
recommended. To avoid this, we manage the update of the autoreload value
in interruption

Signed-off-by: Julien D'Ascenzio <julien.dascenzio@paratronic.fr>