arch: arm: cortex_m: pm_s2ram: wrap context save/restore in macros

Wrap the CPU register save/restore operations (GPR and special registers)
in macros to make core logic simpler to follow. This is also a preparatory
step to introduce ARMv6-M and ARMv7-M support.

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