arm64: dts: meson-gx: cec node should be disabled by default

This fixes the following DT schemas check errors:
meson-gxl-s905x-hwacom-amazetv.dt.yaml: cec@100: 'hdmi-phandle' is a required property
meson-gxm-rbox-pro.dt.yaml: cec@100: 'hdmi-phandle' is a required property

because CEC is not enabled on these boards.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>