arm64: dts: sprd: Add clock properties for serial devices

We've introduced power management logics for the Spreadtrum serial
controller by commit 062ec2774c8a ("serial: sprd: Add power management
for the Spreadtrum serial controller"), thus add related clock properties
to support this feature.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>