Restrict unpriv task to invoke code with privilege

Restrict unpriv task to invoke code with privilege

It was possible for an unprivileged task to invoke any function with
privilege by passing it as a parameter to MPU_xTaskCreate,
MPU_xTaskCreateStatic, MPU_xTimerCreate, MPU_xTimerCreateStatic, or
MPU_xTimerPendFunctionCall.

This commit ensures that MPU_xTaskCreate and MPU_xTaskCreateStatic can
only create unprivileged tasks. It also removes the following APIs:
1. MPU_xTimerCreate
2. MPU_xTimerCreateStatic
3. MPU_xTimerPendFunctionCall

We thank Huazhong University of Science and Technology for reporting
this issue.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>