mgmt: mcumgr: Add access denied error code

Adds an access denied error code that can be used to signal to the
mcumgr client that the requested access to a specific resource or
command/functionality has been denied.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

subsys/mgmt/mcumgr/lib/mgmt/include/mgmt/mgmt.h

@@ -52,6 +52,7 @@ extern "C" {
 #define MGMT_ERR_ENOTSUP	8	/* Command not supported. */
 #define MGMT_ERR_ECORRUPT	9	/* Corrupt */
 #define MGMT_ERR_EBUSY		10	/* Command blocked by processing of other command */
+#define MGMT_ERR_EACCESSDENIED	11	/* Access to specific function or resource denied */
 #define MGMT_ERR_EPERUSER	256
 
 #define MGMT_HDR_SIZE		8