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/arm/mach-davinci/da850_lowlevel.c b/arch/arm/mach-davinci/da850_lowlevel.c
index 07bf19b..6994ded 100644
--- a/arch/arm/mach-davinci/da850_lowlevel.c
+++ b/arch/arm/mach-davinci/da850_lowlevel.c
@@ -290,8 +290,8 @@
 	board_gpio_init();
 
 #if !CONFIG_IS_ENABLED(DM_SERIAL)
-	NS16550_init((NS16550_t)(CONFIG_SYS_NS16550_COM1),
-			CONFIG_SYS_NS16550_CLK / 16 / CONFIG_BAUDRATE);
+	NS16550_init((struct ns16550 *)(CONFIG_SYS_NS16550_COM1),
+		     CONFIG_SYS_NS16550_CLK / 16 / CONFIG_BAUDRATE);
 #endif
 	/*
 	 * Fix Power and Emulation Management Register