drivers: timer: systick: fix off-by-one setting in tickless mode

When the counter reaches zero, it reloads the value in
SYST_RVR on the next clock edge. This means that if the
LOAD value is N, the interrupt ("tick") is triggered
every N+1 cycles. Therefore, when we operate in tickless
mode, and we want to schedule the next timeout, we need
to configure the LOAD value with last_load - 1, in order
to get an event in last_load cycles.

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