doc: Update arg usage for BL2 and BL31 setup functions

Updated the porting guide for the usage of received arguments
in BL2 and BL32 setup functions in case of Arm platform.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ia83a5607fed999819d25e49322b3bfb5db9425c0

docs/getting_started/porting-guide.rst

@@ -1424,7 +1424,7 @@ are platform specific.
 
 On Arm standard platforms, the arguments received are :
 
-    arg0 - Points to load address of HW_CONFIG if present
+    arg0 - Points to load address of FW_CONFIG
 
     arg1 - ``meminfo`` structure populated by BL1. The platform copies
     the contents of ``meminfo`` as it may be subsequently overwritten by BL2.
@@ -1736,6 +1736,10 @@ In Arm standard platforms, the arguments received are :
     which is list of executable images following BL31,
 
     arg1 - Points to load address of SOC_FW_CONFIG if present
+           except in case of Arm FVP platform.
+
+           In case of Arm FVP platform, Points to load address
+           of FW_CONFIG.
 
     arg2 - Points to load address of HW_CONFIG if present