dt-bindings: serial: sprd: Add clocks and clocks-names properties

This patch adds clocks and clocks-names properties, which are used to do
power management for our UART driver.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lanqing Liu <lanqing.liu@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>