documentation: arm: sunxi: add Allwinner H6 documents

Add the current Allwinner H6 datasheet and user manual.

Signed-off-by: Wilken Gottwalt <wilken.gottwalt@linux-addicted.net>
Link: https://lore.kernel.org/r/20201027062408.GA6761@monster.powergraphx.local


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