dt-bindings: mtd: cadence: document required clock-names

The clock-names property is required because the driver requests
the clock by name and not the index.
Update the example to use &clk instead of &nf_clk for the clocks
property to avoid confusion with the clock-names property "nf_clk".

Fixes: 1f05f823a16c (dt-bindings: mtd: cadence: convert cadence-nand-controller.txt to yaml)
Signed-off-by: Niravkumar L Rabara <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Miquel Raynal <[e-mail]>

Documentation/devicetree/bindings/mtd/cdns,hp-nfc.yaml

···
   clocks:
     maxItems: 1
 
+  clock-names:
+    items:
+      - const: nf_clk
+
   dmas:
     maxItems: 1
 
···
   - reg-names
   - interrupts
   - clocks
+  - clock-names
 
 unevaluatedProperties: false
 
···
         #address-cells = <1>;
         #size-cells = <0>;
         interrupts = <GIC_SPI 97 IRQ_TYPE_LEVEL_HIGH>;
-        clocks = <&nf_clk>;
+        clocks = <&clk>;
+        clock-names = "nf_clk";
         cdns,board-delay-ps = <4830>;
 
         nand@0 {