ARM: Factor out reusable psci_cpu_entry

ARM: Factor out reusable psci_cpu_entry

_sunxi_cpu_entry can be converted completely into a reusable
psci_cpu_entry. Tegra124 will use it as well.

As with psci_disable_smp, also the enabling is designed to be overloaded
in cased SMP is not controlled via ACTLR.

CC: Marc Zyngier <[email protected]>
Signed-off-by: Jan Kiszka <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Thierry Reding <[email protected]>
Tested-by: Thierry Reding <[email protected]>
Tested-by: Ian Campbell <[email protected]>
Signed-off-by: Tom Warren <[email protected]>