ARM: PSCI: Make psci_get_cpu_stack_top local to armv7/psci.S

Now that we have a secure data section for storing variables, there
should be no need for platform code to get the stack address.

Make psci_get_cpu_stack_top a local function, as it should only be
used in armv7/psci.S and only by psci_stack_setup.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>