docs(fconf): update device tree binding for FCONF

Added a description for the newly introduced 'ns-load-address' property
in the dtb-registry node of FCONF.

Change-Id: Ief8e8a55a6363fd42b23491d000b097b0c48453b
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/components/fconf/fconf_properties.rst b/docs/components/fconf/fconf_properties.rst
index 5c28a7a..20cc758 100644
--- a/docs/components/fconf/fconf_properties.rst
+++ b/docs/components/fconf/fconf_properties.rst
@@ -30,3 +30,10 @@
     - value type: <u32>
     - Image ID of the configuration.
 
+- ns-load-address [optional]
+    - value type: <u64>
+    - Physical loading base address of the configuration in the non-secure
+      memory.
+      Only needed by those configuration files which require being loaded
+      in secure memory (at load-address) as well as in non-secure memory
+      e.g. HW_CONFIG