We want to be able to add other common code to this function. So change the
driver's version to have an underscore before it, just like
_debug_uart_putc(). Define debug_uart_init() to call this version.

Update all drivers to this new method.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>