arch/arm/core/cortex_m: optional memory section for _cp_context

More complex suspend and resume scheme might require exactly defined
location of this variable due to platform peculiar SW and HW requirement.

DTS zephyr,memory-region node with nodelabel `pm_s2ram` shall be used to
automatic definition of linker section for such objective.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>