dt-bindings: gpu: mali-midgard: Add resets property

The Amlogic ARM Mali Midgard requires reset controls to power on and
software reset the GPU, adds these as optional in the bindings.

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