ARM: dts: imx6ul-pico-dwarf: Use 'clock-frequency'

'clock_frequency' is not a valid property.

Use the correct 'clock-frequency' instead.

Fixes: 47246faf ("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>