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 <[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]>