drm/mediatek: mtk_cec: Switch to register as module_platform_driver

drm/mediatek: mtk_cec: Switch to register as module_platform_driver

In preparation for adding a driver for the new HDMIv2 IP, and
before splitting out the common bits from the HDMI driver, change
the mtk_cec driver from being registered from the mtk_hdmi driver
itself to be a module_platform_driver of its own.

Besides being a cleanup, this also allows build flexibility by
allowing to compile the CECv1 driver only when needed (for example,
this is not needed nor used in HDMIv2 configurations).

Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20250415104321.51149-5-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>