ARM: dts: imx6ul-pico-dwarf: Use 'clock-frequency'
'clock_frequency' is not a valid property.
Use the correct 'clock-frequency' instead.
Fixes: 47246fafef84 ("ARM: dts: imx6ul-pico: Add support for the dwarf baseboard")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
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