sunxi: Add base address for GIC

Instead of hardcoding the GIC addresses in the PSCI implementation,
provide a base address in the cpu header.

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>