sunxi: Move remaining PSCI assembly code to C

This patch finishes the rewrite of sunxi specific PSCI parts into C
code.

The assembly-only stack setup code has been factored out into a common
function for ARMv7. The GIC setup code can be renamed as psci_arch_init.
And we can use an empty stub function for psci_text_end.

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