nios2: 10m50: change to ns16550 uart

Change to ns16550 uart for 10m50 devboard based on a new
Altera release.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>