sunxi: Add PSCI implementation in C

To make the PSCI backend more maintainable and easier to port to newer
SoCs, rewrite the current PSCI implementation in C.

Some inline assembly bits are required to access coprocessor registers.
PSCI stack setup is the only part left completely in assembly. In theory
this part could be split out of psci_arch_init into a separate common
function, and psci_arch_init could be completely in C.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>