dt-bindings: amlogic: add unstable statement

Due to the lack of documentation on this SoC family, discovery of new
features, and correcting of previous misunderstandings is expected, so
it's unrealistic to expect any form of stable bindings for this SoC
family.  Make that clear inthe binding documentation.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>