arm64: dts: meson: libretech-pc: add internal DAC support

Add the internal DAC support on the libretech CC. The output of this
DAC is provided on the 9J4 connector.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200506221656.477379-6-jbrunet@baylibre.com