arm64: dts: imx8mq-kontron-pitx-imx8m-u-boot.dtsi: disable assigned clocks

With the move to use DM_CLK the boards uart stops working. The used
properties are not supported by the imx8mq clock driver. Thus
the correct baudrate cannot be selected. Remove this properties here and
the board can start with working uart. Keep it in the main dts because
linux handles these porperties fine.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>