dt-bindings: clock: ingenic: Explicitly list compatible strings

This is better than letting the other developers wondering what are the
supported strings.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/ingenic,cgu.txt

@@ -6,8 +6,11 @@ to provide many different clock signals derived from only 2 external source
 clocks.
 
 Required properties:
-- compatible : Should be "ingenic,<soctype>-cgu".
-  For example "ingenic,jz4740-cgu" or "ingenic,jz4780-cgu".
+- compatible : Should be one of:
+  * ingenic,jz4740-cgu
+  * ingenic,jz4725b-cgu
+  * ingenic,jz4770-cgu
+  * ingenic,jz4780-cgu
 - reg : The address & length of the CGU registers.
 - clocks : List of phandle & clock specifiers for clocks external to the CGU.
   Two such external clocks should be specified - first the external crystal