ARM: sunxi: Add sun6i specific PSCI implementation

This adds PSCI support for sun6i. So far it only supports
the PWR_ON method.

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