psci: arm: remove armv7 function psci_save_target_pc

This function is no more used, and replaced by psci_save
which save also context id as requested by PSCI requirements.

Even if the context id is not used by Linux, it should be saved
and restored in r0 when the CPU_ON is performed.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>