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/arch/x86/cpu/apollolake/uart.c b/arch/x86/cpu/apollolake/uart.c
index 26aef65..8e6dfdb 100644
--- a/arch/x86/cpu/apollolake/uart.c
+++ b/arch/x86/cpu/apollolake/uart.c
@@ -127,7 +127,7 @@
 	.id	= UCLASS_SERIAL,
 	.of_match = apl_ns16550_serial_ids,
 	.plat_auto	= sizeof(struct ns16550_plat),
-	.priv_auto	= sizeof(struct NS16550),
+	.priv_auto	= sizeof(struct ns16550),
 	.ops	= &ns16550_serial_ops,
 	.of_to_plat = apl_ns16550_of_to_plat,
 	.probe = apl_ns16550_probe,
diff --git a/arch/x86/cpu/slimbootloader/serial.c b/arch/x86/cpu/slimbootloader/serial.c
index 5d89632..ebbd2c5 100644
--- a/arch/x86/cpu/slimbootloader/serial.c
+++ b/arch/x86/cpu/slimbootloader/serial.c
@@ -59,7 +59,7 @@
 	.of_match = slimbootloader_serial_ids,
 	.of_to_plat = slimbootloader_serial_of_to_plat,
 	.plat_auto	= sizeof(struct ns16550_plat),
-	.priv_auto	= sizeof(struct NS16550),
+	.priv_auto	= sizeof(struct ns16550),
 	.probe	= ns16550_serial_probe,
 	.ops	= &ns16550_serial_ops,
 };