cpu/stm32: add stm32mp157x vendor headers

Add vendor CMSIS headers from STMicroelectronics:
https://wiki.st.com/stm32mpu/wiki/STM32CubeMP1_architecture#CMSIS

Signed-off-by: Gilles DOFFE <gilles.doffe@savoirfairelinux.com>

cpu/stm32/include/vendor/system_stm32mp1xx.h

@@ -0,0 +1,106 @@
+/**
+  ******************************************************************************
+  * @file    system_stm32mp1xx.h
+  * @author  MCD Application Team
+  * @brief   CMSIS Cortex-Mx Device System Source File for STM32MP1xx devices.
+  ******************************************************************************
+  * @attention
+  *
+  * <h2><center>&copy; Copyright (c) 2019 STMicroelectronics.
+  * All rights reserved.</center></h2>
+  *
+  * This software component is licensed by ST under BSD 3-Clause license,
+  * the "License"; You may not use this file except in compliance with the
+  * License. You may obtain a copy of the License at:
+  *                        opensource.org/licenses/BSD-3-Clause
+  *
+  ******************************************************************************
+  */
+
+/** @addtogroup CMSIS
+  * @{
+  */
+
+/** @addtogroup stm32mp1xx_system
+  * @{
+  */
+
+/**
+  * @brief Define to prevent recursive inclusion
+  */
+#ifndef __SYSTEM_STM32MP1XX_H
+#define __SYSTEM_STM32MP1XX_H
+
+#ifdef __cplusplus
+ extern "C" {
+#endif
+
+
+/** @addtogroup STM32MP1xx_System_Includes
+  * @{
+  */
+
+/**
+  * @}
+  */
+
+
+/** @addtogroup STM32MP1xx_System_Exported_types
+  * @{
+  */
+  /* This variable is updated in three ways:
+      1) by calling CMSIS function SystemCoreClockUpdate()
+      2) by calling HAL API function HAL_RCC_GetSysClockFreq()
+      3) each time HAL_RCC_ClockConfig() is called to configure the system clock frequency
+         Note: If you use this function to configure the system clock; then there
+               is no need to call the 2 first functions listed above, since SystemCoreClock
+               variable is updated automatically.
+  */
+extern uint32_t SystemCoreClock;          /*!< System Core1 Clock Frequency  */
+extern uint32_t SystemCore1Clock;         /*!< System Core1 Clock Frequency  */
+extern uint32_t SystemCore2Clock;         /*!< System Core2 Clock Frequency  */
+
+/**
+  * @}
+  */
+
+/** @addtogroup STM32MP1xx_System_Exported_Constants
+  * @{
+  */
+
+/**
+  * @}
+  */
+
+/** @addtogroup STM32MP1xx_System_Exported_Macros
+  * @{
+  */
+
+/**
+  * @}
+  */
+
+/** @addtogroup STM32MP1xx_System_Exported_Functions
+  * @{
+  */
+
+extern void SystemInit(void);
+extern void SystemCoreClockUpdate(void);
+/**
+  * @}
+  */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /*__SYSTEM_STM32MP1XX_H */
+
+/**
+  * @}
+  */
+
+/**
+  * @}
+  */
+/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/