debug_uart: restore ns16550 as default

debug_uart: restore ns16550 as default

Since commit 220e8021af96 ("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>