serial: Update NS16550_t and struct NS16550

Typedefs should not be used in U-Boot and structs should be lower case.
Update the code to use struct ns16550 consistently.

Put a header guard on the file while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
diff --git a/drivers/serial/serial_rockchip.c b/drivers/serial/serial_rockchip.c
index 4c0548e..036c072 100644
--- a/drivers/serial/serial_rockchip.c
+++ b/drivers/serial/serial_rockchip.c
@@ -42,7 +42,7 @@
 U_BOOT_DRIVER(rockchip_rk3188_uart) = {
 	.name	= "rockchip_rk3188_uart",
 	.id	= UCLASS_SERIAL,
-	.priv_auto	= sizeof(struct NS16550),
+	.priv_auto	= sizeof(struct ns16550),
 	.plat_auto	= sizeof(struct rockchip_uart_plat),
 	.probe	= rockchip_serial_probe,
 	.ops	= &ns16550_serial_ops,
@@ -52,7 +52,7 @@
 U_BOOT_DRIVER(rockchip_rk3288_uart) = {
 	.name	= "rockchip_rk3288_uart",
 	.id	= UCLASS_SERIAL,
-	.priv_auto	= sizeof(struct NS16550),
+	.priv_auto	= sizeof(struct ns16550),
 	.plat_auto	= sizeof(struct rockchip_uart_plat),
 	.probe	= rockchip_serial_probe,
 	.ops	= &ns16550_serial_ops,