arch: arc: modify the kconfig for mpu case

* user space support requires THREAD_INFO
* for MPU version 2, the stack align is at least 2048 bytes
  * the smallest mpu region is 2048 bytes
  * the region size must bt power of 2
  * the start address of region must be aligned to the region size

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>