sunxi: use vendor_device scheme for device definitions

This changes device definition to resemble the vendor_device scheme
already present for the majority of device compatible strings.

By doing this, we achieve several advantages at once:
- Image names and node names are more consistent with other targets.
- SUPPORTED_DEVICES can be set automatically for all but two cases.
- Image names and node names are in line with DEVICE_TITLEs.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>