arch: arm: cortex_m: pm_s2ram: add support for all architectures

Extend the ARM M-profile suspend-to-RAM implementation to be compatible
with all versions of the M-profile supported by Zephyr: ARMv6-M, ARMv7-M,
and ARMv8-M Baseline.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>