Currently, the serial nodes define both a clock-frequency and a clocks
property. We should not provide both, since they might conflict.

In practice, this also causes problems since the of_serial driver uses
the clock-frequency property in preference to the clocks property, and
hence doesn't clk_prepare_enable() the clock, which may then leave it
with no known users, and hence the common clock framework will disable
it, thus breaking the port, which is usually the console.
Signed-off-by: NStephen Warren <swarren@nvidia.com>