arm64: dts: actions: s900: Source CMU clock for UARTs

Remove fixed clock in Bubblegum-96 board and source CMU (Clock
Management Unit) clock for UART nodes in Actions Semi S900 SoC.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[AF: Move/add clocks to SoC]
Signed-off-by: Andreas Färber <afaerber@suse.de>