sunxi: Add Pine64+ support

The Pine64+ is a system based on the Allwinner A64 SoC. It is capable of
running AArch64 code and thus is the first of its kind for the sunxi target.

This patch adds a defconfig and device tree chunks for it.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
[agraf: Change patch description]
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>