riscv: dts: mpfs-icicle-kit: Drop 'clock-frequency' in the uart nodes

The uart nodes already provide <clocks> property for the driver to
dynamically calculate the correct clock frequency. There is no need
to keep the hard-coded <clock-frequency> property.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Padmarao Begari <padmarao.begari@microchip.com>
Tested-by: Padmarao Begari <padmarao.begari@microchip.com>