arch: arm: cortex_m: Modifed FPU save and restore helpers

z_arm_save_fp_context and z_arm_restore_fp_context
save and restore fpu context regardless of the
CONFIG_FPU_SHARING setting.

This modification is required to support suspend to ram use cases
where save and restore of FPU state is needed to ensure proper bahaviour
after wakeup.

Signed-off-by: Michele Sardo <michele.sardo@st.com>