ACP transactions can cause coherent requests to the system.
Therefore ACP requests must pass the Inner and Outer Shareable attributes
to the L2. To pass the Inner Shareable attribute, use ARUSER and AWUSER.
To pass the Outer Shareable attribute, use ARUSER and AWUSER.
The setting of AxUSER[1:0] to
0b11 is not allowed and causes a SLVERR response.