ns16550: zap CONFIG_NS16550_SERIAL

Zap CONFIG_NS16550_SERIAL, as the unification of ns16550 drivers
is completed.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
16 files changed