dt-bindings: media: amlogic,vdec: add default compatible

The first version of the bindings is missing a generic compatible that
is used by the base node (GX), and then extended by the SoC device trees
(GXBB, GXL, GXM)

Also change the example to use "video-codec" instead of "video-decoder",
as the former is the one used in almost all cases when it comes to video
decode/encode accelerators.

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