Update spm.md

Apply code review comments

Author: danny4478

docs/porting/psa/spm.md

@@ -108,15 +108,16 @@ This table describes the allowed operations (Read / Write / Execute) on the Secu
 - X means No Access
 - V means Must Be Able to Access
 - ? means it is up to the target
+- X? means it is up to the target, preferably No Access
 
 Processor Access    |Secure RAM        |Secure FLASH|Non Secure RAM     |Non Secure FLASH
 --------------------|------------------|------------|-------------------|----------------
 `Non Secure Read`   |   X              |    X       |        V          |    V
 `Non Secure Write`  |   X              |    X       |        V          |    ?
-`Non Secure Execute`|   X              |    X       |  ? (preferably X) |    V
+`Non Secure Execute`|   X              |    X       |        X?         |    V
 `Secure Read`       |   V              |    V       |        V          |    V
 `Secure Write`      |   V              |    V       |        V          |    ?
-`Secure Execute`    | ? (preferably X) |    V       |        X          |    ?
+`Secure Execute`    |   X?             |    V       |        X          |    ?
 
 
 ## Testing