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/plat/hisilicon/hikey/include/hi6220_regs_ao.h b/plat/hisilicon/hikey/include/hi6220_regs_ao.h
index 79a5404..132f33c 100644
--- a/plat/hisilicon/hikey/include/hi6220_regs_ao.h
+++ b/plat/hisilicon/hikey/include/hi6220_regs_ao.h
@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#ifndef __HI6220_AO_H__
-#define __HI6220_AO_H__
+#ifndef HI6220_REGS_AO_H
+#define HI6220_REGS_AO_H
 
 #define AO_CTRL_BASE				0xF7800000
 
@@ -331,4 +331,4 @@
 #define PCLK_TIMER1						(1 << 16)
 #define PCLK_TIMER0						(1 << 15)
 
-#endif /* __HI6220_AO_H__ */
+#endif /* HI6220_REGS_AO_H */