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: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NTom Rini <trini@konsulko.com>
Acked-by: NSimon Glass <sjg@chromium.org>

drivers/serial/serial-uclass.c

@@ -16,8 +16,6 @@
 #include <dm/lists.h>
 #include <dm/device-internal.h>
 
-#include <ns16550.h>
-
 DECLARE_GLOBAL_DATA_PTR;
 
 /*