ARM64: dts: meson-gxbb-nanopi-k2: Add HDMI, CEC and CVBS nodes

The Amlogic Meson GXBB based Nanopi-K2 board has an HDMI connector
with CEC and CVBS available on the 40pin header.
This patch adds the nodes to enable HDMI, CEC and CVBS functionnalities.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>