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 <marc.zyngier@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Tested-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Tom Warren <twarren@nvidia.com>