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/bl2/bl2_private.h b/bl2/bl2_private.h
index f93a179..7fd17bf 100644
--- a/bl2/bl2_private.h
+++ b/bl2/bl2_private.h
@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#ifndef __BL2_PRIVATE_H__
-#define __BL2_PRIVATE_H__
+#ifndef BL2_PRIVATE_H
+#define BL2_PRIVATE_H
 
 #if BL2_IN_XIP_MEM
 /*******************************************************************************
@@ -33,4 +33,4 @@
 struct entry_point_info *bl2_load_images(void);
 void bl2_run_next_image(const struct entry_point_info *bl_ep_info);
 
-#endif /* __BL2_PRIVATE_H__ */
+#endif /* BL2_PRIVATE_H */