ARM: sunxi: Make PSCI code sun7i specific

The PSCI code only works for sun7i. Rename it with _sun7i suffix,
and build only if building for sun7i.

This paves the way for adding PSCI support for other platforms.

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>