ARM: Factor out reusable psci_get_cpu_stack_top

ARM: Factor out reusable psci_get_cpu_stack_top

This algorithm will be useful on Tegra as well, plus we will need it for
making _psci_target_pc per-CPU.

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>