ARM: PSCI: Split out common stack setup code from psci_arch_init

Every platform has the same stack setup code in assembly as part of
psci_arch_init.

Move this out into a common separate function, psci_stack_setup, for
all platforms. This will allow us to move the remaining parts of
psci_arch_init into C code, or drop it entirely.

Also provide a stub no-op psci_arch_init for platforms that don't need
their own specific setup code.

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