drivers: timer: systick: fix off-by-one setting

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 non-
tickless mode, we need to configure the LOAD value
with CYC_PER_TICK - 1, in order to get an event
every CYC_PER_TICK cycles.

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