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: 1f05f823 (dt-bindings: mtd: cadence: convert cadence-nand-controller.txt to yaml)
Signed-off-by: Niravkumar L Rabara <niravkumar.l.rabara@intel.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>