clk: shmobile: rz: Document mandatory compatible fallback

The generic RZ 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,rz-cpg-clocks.txt

@@ -7,7 +7,7 @@ Required Properties:
 
   - compatible: Must be one of
     - "renesas,r7s72100-cpg-clocks" for the r7s72100 CPG
-    - "renesas,rz-cpg-clocks" for the generic RZ CPG
+    and "renesas,rz-cpg-clocks" as a fallback.
   - reg: Base address and length of the memory resource used by the CPG
   - clocks: References to possible parent clocks. Order must match clock modes
     in the datasheet. For the r7s72100, this is extal, usb_x1.