boards: nordic: Enable CONFIG_HW_STACK_PROTECTION conditionally

All Nordic boards were enabling CONFIG_HW_STACK_PROTECTION
unconditionally, which is usually not a problem except if the particular
application happens to want to disable the MPU (or equivalent hardware
for RISC-V). In that case, the following warning is shown:

warning: HW_STACK_PROTECTION (defined at arch/Kconfig) was assigned the
value 'y' but got the value 'n'.
Check these unsatisfied dependencies: ARCH_HAS_STACK_PROTECTION (=n).

In order to avoid this warning, conditionally enable the hardware stack
protection only when the architecture supports it.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>