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>

include/drivers/st/stm32_gpio.h

@@ -4,8 +4,8 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
-#ifndef __PLAT_GPIO_H__
-#define __PLAT_GPIO_H__
+#ifndef STM32_GPIO_H
+#define STM32_GPIO_H
 
 #include <utils_def.h>
 
@@ -98,4 +98,4 @@ void set_gpio(uint32_t bank, uint32_t pin, uint32_t mode, uint32_t speed,
 	      uint32_t pull, uint32_t alternate);
 #endif /*__ASSEMBLY__*/
 
-#endif /*__PLAT_GPIO_H__*/
+#endif /* STM32_GPIO_H */