debug_uart: restore ns16550 as default

Since commit 220e8021af ("nios2: convert altera_jtag_uart to
driver model"), the default debug uart was changed. Most people
use ns16550 UART, so restore it as default.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reported-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reported-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>

drivers/serial/Kconfig

@@ -53,6 +53,7 @@ config DEBUG_UART
 choice
 	prompt "Select which UART will provide the debug UART"
 	depends on DEBUG_UART
+	default DEBUG_UART_NS16550
 
 config DEBUG_UART_ALTERA_JTAGUART
 	bool "Altera JTAG UART"