dt-bindings: arm: amlogic: add G12B bindings

Add compatible for the Amlogic G12B SoC, sharing most of the
features and architecture with the G12A SoC.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>