arm: psci: add a weak function psci_arch_cpu_entry

The added function psci_arch_cpu_entry() is called
during psci_cpu_entry() and can be used by arch to handle
PSCI state transition from ON_PENDING to ON.

The default weak function is empty: not behavior change.

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