ARM: dts: imx6sx-nitrogen6sx: drop incorrect regulator clock-names

regulator-fixed does not take "clock-names" property:

  imx6sx-nitrogen6sx.dtb: regulator-wlan: Unevaluated properties are not allowed ('clock-names' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>