arch: arm: cortex_m: pm_s2ram: ignore xPSR

Remove all xPSR-related registers from struct __cpu_context, and the
associated save/restore code in S2RAM code, as they are not needed:

* EPSR and IPSR are read-only - they cannot be "restored"
* Bits N, V, Z, C, V, Q, and GE (if DSP Extension is implemented) of APSR
  could be restored, but this is not needed as the AAPCS indicates these
  bits to be "undefined on entry to or return from a public interface"

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