docs: change all occurrences of RSS to RSE

Changes all occurrences of "RSS" and "rss" in the documentation
to "RSE" and "rse".

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: Ia42078f5faa1db331b1e5a35f01faeaf1afacb5f
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index 7c66d11..fdd4340 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -256,10 +256,10 @@
 
    Defines the maximum address in secure RAM that the BL31 image can occupy.
 
--  **#define : PLAT_RSS_COMMS_PAYLOAD_MAX_SIZE**
+-  **#define : PLAT_RSE_COMMS_PAYLOAD_MAX_SIZE**
 
-   Defines the maximum message size between AP and RSS. Need to define if
-   platform supports RSS.
+   Defines the maximum message size between AP and RSE. Need to define if
+   platform supports RSE.
 
 For every image, the platform must define individual identifiers that will be
 used by BL1 or BL2 to load the corresponding image into memory from non-volatile