dt-bindings: gpu: mali-utgard: add Amlogic Meson8 and Meson8b compatible

Add a compatible string for the Mali-450 GPU on Amlogic Meson8 and
Meson8b SoCs. Meson8 uses an "MP6" variant with six pixel processors
while Meson8b (as cost-reduced SoC) uses an "MP2" variant with two pixel
processors. Both have a reset line to bring the GPU into a well-defined
state.

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