drivers: timer: systick: clear pending events when resetting timeout

In the unlucky scenario of a SysTick event (wrap) occurring
while we re-program the last_load value, the SysTick ISR
will run immediately after we unlock interrupts. In that
case the timeout we have just configured will expire
instantaneously, leading to operations being executed
much earlier than expected. Avoid this by clearing possibly
pending SysTick exceptions (writing 1 to ICSR.PENDSTCLR).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>