arch: arm: cortex-m: Modify ARM-only API for IRQ target state mgmt

we modify the ARM Cortex-M only API for managing the
security target state of the NVIC IRQs. We remove the
internal ASSERT checking allowing to call the API for
non-implemented NVIC IRQ lines. However we still give the
option to the user to check the success of the IRQ target
state setting operation by allowing the API function to
return the resulting target state.

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