dt-bindings: clock: versaclock3: Add description for #clock-cells property

Add description for "#clock-cells" property to map indexes to the clock
output in the Table 3. ("Output Source") in the 5P35023 datasheet
(ie: {REF,SE1,SE2,SE3,DIFF1,DIFF2}. Also update the "assigned-clock-rates"
in the example.

While at it, replace clocks phandle in the example from x1_x2->x1 as
X2 is a different 32768 kHz crystal.

Suggested-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Biju Das <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/renesas,5p35023.yaml

···
     maxItems: 1
 
   '#clock-cells':
+    description:
+      The index in the assigned-clocks is mapped to the output clock as below
+      0 - REF, 1 - SE1, 2 - SE2, 3 - SE3, 4 - DIFF1, 5 - DIFF2.
     const: 1
 
   clocks:
···
             reg = <0x68>;
             #clock-cells = <1>;
 
-            clocks = <&x1_x2>;
+            clocks = <&x1>;
 
             renesas,settings = [
                 80 00 11 19 4c 02 23 7f 83 19 08 a9 5f 25 24 bf
···
             assigned-clocks = <&versa3 0>, <&versa3 1>,
                               <&versa3 2>, <&versa3 3>,
                               <&versa3 4>, <&versa3 5>;
-            assigned-clock-rates = <12288000>, <25000000>,
-                                   <12000000>, <11289600>,
-                                   <11289600>, <24000000>;
+            assigned-clock-rates = <24000000>, <11289600>,
+                                   <11289600>, <12000000>,
+                                   <25000000>, <12288000>;
         };
     };