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>

Documentation/devicetree/bindings/media/amlogic,vdec.txt

@@ -26,6 +26,7 @@ Required properties:
 	- GXBB (S905) : "amlogic,gxbb-vdec"
 	- GXL (S905X, S905D) : "amlogic,gxl-vdec"
 	- GXM (S912) : "amlogic,gxm-vdec"
+	followed by the common "amlogic,gx-vdec"
 - reg: base address and size of he following memory-mapped regions :
 	- dos
 	- esparser
@@ -47,8 +48,8 @@ Required properties:
 
 Example:
 
-vdec: video-decoder@c8820000 {
-	compatible = "amlogic,gxbb-vdec";
+vdec: video-codec@c8820000 {
+	compatible = "amlogic,gxbb-vdec", "amlogic,gx-vdec";
 	reg = <0x0 0xc8820000 0x0 0x10000>,
 	      <0x0 0xc110a580 0x0 0xe4>;
 	reg-names = "dos", "esparser";