dt-bindings: power: Add MT8196 GPU frequency control binding

dt-bindings: power: Add MT8196 GPU frequency control binding

On the MT8196 and MT6991 SoCs, the GPU power and frequency is controlled
by some integration logic, referred to as "MFlexGraphics" by MediaTek,
which comes in the form of an embedded controller running
special-purpose firmware.

This controller takes care of the regulators and PLL clock frequencies
to squeeze the maximum amount of power out of the silicon.

Add a binding which models it as a power domain.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>