documentation: arm: sunxi: add Allwinner H616 documents

Add the current and cleaned Allwinner H616 datasheet and user manual.

Signed-off-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/20201108150104.GA66507@monster.powergraphx.local


Signed-off-by: Jonathan Corbet <corbet@lwn.net>