rk3399: Don't enable the debug UART if there is no driver

Some boards do not enable SPL_SERIAL so cannot use the debug UART. Add
this condition to the code and drop use of the preprocessor while we are
here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>