Standardise header guards across codebase

All identifiers, regardless of use, that start with two underscores are
reserved. This means they can't be used in header guards.

The style that this project is now to use the full name of the file in
capital letters followed by 'H'. For example, for a file called
"uart_example.h", the header guard is UART_EXAMPLE_H.

The exceptions are files that are imported from other projects:

- CryptoCell driver
- dt-bindings folders
- zlib headers

Change-Id: I50561bf6c88b491ec440d0c8385c74650f3c106e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/include/drivers/ti/uart/uart_16550.h b/include/drivers/ti/uart/uart_16550.h
index 9ee2a33..ad80c57 100644
--- a/include/drivers/ti/uart/uart_16550.h
+++ b/include/drivers/ti/uart/uart_16550.h
@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#ifndef __UART_16550_H__
-#define __UART_16550_H__
+#ifndef UART_16550_H
+#define UART_16550_H
 
 #include <console.h>
 
@@ -93,4 +93,4 @@
 
 #endif /*__ASSEMBLY__*/
 
-#endif	/* __UART_16550_H__ */
+#endif /* UART_16550_H */