rockchip: Turn on CONFIG_DEBUG_UART_NS16550 in defconfigs

No UART driver was specified in defconfig, thus
DEBUG_UART_ALTERA_JTAGUART was incorrectly selected by default since
commit 220e8021 added a new Altera UART
driver.

Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Acked-by: Simon Glass <sjg@chromium.org>