最大8个region，每个region可独立配置（先配置region numuber，再配置RBAR，RASR）.
可覆盖,高region num的region 覆盖低 region num的region配置.
size 必须是2的n次方.
base address必须是size的n倍.....
The MPU divides the memory map into a number of regions, and defines the location, size,
access permissions, and memory attributes of each region. It supports:
• independent attribute settings for each region
• overlapping regions
• export of memory attributes to the system
The memory attributes affect the behavior of memory accesses to the region. The Cortex-M4
MPU defines:
• eight separate memory regions, 0-7
• a background region.
