dm: serial: remove bogus include <ns16550.h>

Serial-uclass should be generically implemented without depending
a particular hardware.  Fortunately, nothing in include/ns16550.h is
referenced from drivers/serial/serial-uclass.c, so remove this bogus
include.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>