dt-bindings: arm: amlogic: Add NanoPi K2

The FriendlyARM NanoPi K2 is a single-board computer.

Cc: techsupport@friendlyarm.com
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>