arm: ensure SysTick IRQ level is set unconditionally

When the SoC implements SysTick, but the system
does not use it as the driver for system timing
we still need to set its interrupt level. This
is because the SysTick IRQ is always enabled,
so we must ensure the interrupt priority is set
to a level lower than the kernel interrupts (for
the assert mechanism to work properly) in case
the SysTick interrupt is accidentaly raised.

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