doc: security/etsi: Provision 5.3.15 assessment

Add self-assessment for provision 5.3.15

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
diff --git a/doc/security/standards/etsi-303645.rst b/doc/security/standards/etsi-303645.rst
index c0376c6..f829f7a 100644
--- a/doc/security/standards/etsi-303645.rst
+++ b/doc/security/standards/etsi-303645.rst
@@ -373,7 +373,7 @@
       - For constrained devices that cannot have their software updated, the product
         should be isolable and the hardware replaceable.
       - R C
-      -
+      - N/A
       -
 
         .. _ETSI_Provision_5_3_16: