pmdomain: mediatek: Add support for MFlexGraphics

pmdomain: mediatek: Add support for MFlexGraphics

Various MediaTek SoCs use GPU integration silicon named "MFlexGraphics"
by MediaTek. On the MT8196 and MT6991 SoCs, interacting with this
integration silicon is required to power on the GPU.

This glue silicon is in the form of an embedded microcontroller running
special-purpose firmware, which autonomously adjusts clocks and
regulators.

Implement a driver, modelled as a pmdomain driver with a
set_performance_state operation, to support these SoCs.

The driver also exposes the actual achieved clock rate, as read back
from the MCU, as common clock framework clocks, by acting as a clock
provider as well.

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>