stm32/mpu: Define the last used MPU region number.

The reason for this change is that it makes allows custom code, that needs
to use an MPU region, to find a free one by using this macro or starting
from the max number and downwards, without concern that it might change in
the future.

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

ports/stm32/mpu.h

@@ -36,12 +36,18 @@
 #define MPU_REGION_QSPI3    (MPU_REGION_NUMBER3)
 #define MPU_REGION_SDRAM1   (MPU_REGION_NUMBER4)
 #define MPU_REGION_SDRAM2   (MPU_REGION_NUMBER5)
-#define MPU_REGION_OPENAMP  (MPU_REGION_NUMBER15)
 
 // Only relevant on CPUs with D-Cache, must be higher priority than SDRAM
 #define MPU_REGION_DMA_UNCACHED_1 (MPU_REGION_NUMBER6)
 #define MPU_REGION_DMA_UNCACHED_2 (MPU_REGION_NUMBER7)
 
+#ifdef MPU_REGION_NUMBER8
+#define MPU_REGION_OPENAMP  (MPU_REGION_NUMBER8)
+#define MPU_REGION_LAST_USED (MPU_REGION_NUMBER8)
+#else
+#define MPU_REGION_LAST_USED (MPU_REGION_NUMBER7)
+#endif
+
 // Attribute value to disable a region entirely, remove it from the MPU
 // (i.e. the MPU_REGION_ENABLE bit is unset.)
 #define MPU_CONFIG_DISABLE 0
@@ -135,6 +141,7 @@ static inline void mpu_config_end(uint32_t irq_state) {
 
 #define MPU_REGION_SIG      (MPU_REGION_NUMBER0)
 #define MPU_REGION_ETH      (MPU_REGION_NUMBER1)
+#define MPU_REGION_LAST_USED (MPU_REGION_NUMBER1)
 
 #define ST_DEVICE_SIGNATURE_BASE (0x08fff800)
 #define ST_DEVICE_SIGNATURE_LIMIT (0x08ffffff)