clk: mediatek: Add MT8196 disp-ao clock support

Add support for the MT8196 disp-ao clock controller, which provides
clock gate control for the display system. It is integrated with the
mtk-mmsys driver, which registers the disp-ao clock driver via
platform_device_register_data().

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Laura Nao <laura.nao@collabora.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>