arm64: dts: actions: s700: Set UART clock references from CMU

Remove fixed clock in Cubieboard 7 and use Clock Management Unit clocks
for all UART nodes in Actions Semi S700 SoC.

Signed-off-by: Parthiban Nallathambi <pn@denx.de>
Signed-off-by: Saravanan Sekar <sravanhome@gmail.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[AF: Moved/added to SoC]
Signed-off-by: Andreas Färber <afaerber@suse.de>