arm: dts: owl-s500: Set CMU clocks for UARTs

Set Clock Management Unit clocks for the UART nodes of Actions Semi
S500 SoCs and remove the dummy "uart2_clk" and "uart3_clk" fixed clocks.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>