arm64: dts: imx8mm-nitrogen-r2: fix WM8960 clock name

The WM8960 Linux driver expects the clock to be named "mclk".  Otherwise
the clock will be ignored and not prepared/enabled by the driver.

Fixes: 40ba2eda ("arm64: dts: imx8mm-nitrogen-r2: add audio")
Cc: <stable@vger.kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>