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/io/io_fip.h b/include/drivers/io/io_fip.h
index 1131cc7..e0b5746 100644
--- a/include/drivers/io/io_fip.h
+++ b/include/drivers/io/io_fip.h
@@ -4,11 +4,11 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#ifndef __IO_FIP_H__
-#define __IO_FIP_H__
+#ifndef IO_FIP_H
+#define IO_FIP_H
 
 struct io_dev_connector;
 
 int register_io_dev_fip(const struct io_dev_connector **dev_con);
 
-#endif /* __IO_FIP_H__ */
+#endif /* IO_FIP_H */