clk: shmobile: rcar-gen2: Document mandatory compatible fallback

The generic R-Car Gen2 CPG compatible value is mandatory, as the driver
uses only this value for matching.  Document that this is a fallback
that must be present.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Documentation/devicetree/bindings/clock/renesas,rcar-gen2-cpg-clocks.txt

@@ -10,7 +10,7 @@ Required Properties:
     - "renesas,r8a7791-cpg-clocks" for the r8a7791 CPG
     - "renesas,r8a7793-cpg-clocks" for the r8a7793 CPG
     - "renesas,r8a7794-cpg-clocks" for the r8a7794 CPG
-    - "renesas,rcar-gen2-cpg-clocks" for the generic R-Car Gen2 CPG
+    and "renesas,rcar-gen2-cpg-clocks" as a fallback.
 
   - reg: Base address and length of the memory resource used by the CPG