sunxi: Reserve ATF memory space on A64

On the A64 we usually boot with ATF running in EL3. ATF as it is available
today resides in the first 16MB of RAM. So we should make sure we reserve
that space in our memory maps.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>