Remove fixed clock and source common clock for UART controllers.

Link: https://lore.kernel.org/r/20200114040311.6599-3-manivannan.sadhasivam@linaro.orgSigned-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>