soc: arm: introduce Kconfig option for Nordic IDAU

This commit introduces a new Kconfig symbol to signify
that a CPU implements the Nordic-specific security
attribution unit. The symbol is meant to be selected
by Nordic SoCs that implement the System Protection
Unit (SPU) peripheral.

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