drm/mediatek: mtk_hdmi_common: Make CEC support optional

drm/mediatek: mtk_hdmi_common: Make CEC support optional

In preparation for adding a new driver for HDMIv2, for which CEC
is not strictly required, change the of_get_compatible_child()
failure error to -EOPNOTSUPP to be able to differentiate between
error conditions in mtk_hdmi_dt_parse_pdata().

In that case, if -EOPNOTSUPP is returned, this driver will print
an informative message saying that CEC support is unavailable,
as the devicetree node for that was not found, but after that,
function mtk_hdmi_dt_parse_pdata() will not return error to
the caller.

This will not change functionality of the mtk_hdmi (v1) driver
as that is still checking whether CEC is present and, if not,
will fail probing with an error saying that CEC is required
by HDMIv1.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20251023-mediatek-drm-hdmi-v2-v11-5-7873ec4a1edf@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>