arm/ffa: deviate violation of MISRA C Rule 20.12

MISRA C Rule 20.12 states: "A macro parameter used as an operand to
the # or ## operators, which is itself subject to further macro replacement,
shall only be used as an operand to these operators".

In this case the 'FFA_*' macros are used both as a regular macro argument and
as an operand for '#' stringification in the expansion of 'FW_ABI()'.
This is deviated using a SAF-x-safe comment.

No functional changes.

Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/tee/ffa.c

@@ -80,6 +80,7 @@ struct ffa_fw_abi {
     const char *name;
 };
 
+/* SAF-6-safe Rule 20.12 expansion of macro FFA_* with FW_ABI */
 #define FW_ABI(abi) {abi,#abi}
 
 /* List of ABI we use from the firmware */