ARM: dts: imx7d-pico: Use 'clock-frequency'

'clock_frequency' is not a valid property.

Use the correct 'clock-frequency' instead.

Fixes: 8b646cfb ("ARM: dts: imx7d-pico: Add support for the dwarf baseboard")
Fixes: 6418fd92 ("ARM: dts: imx7d-pico: Add support for the nymph baseboard")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>