Modify return type of plat_get_ns_image_entrypoint()

This patch modifies the return type of the platform API
`plat_get_ns_image_entrypoint()` from `unsigned long` to
`uintptr_t` in accordance with the coding guidelines.

Change-Id: Icb4510ca98b706aa4d535fe27e203394184fb4ca
diff --git a/docs/porting-guide.md b/docs/porting-guide.md
index f0adf57..f1f14a8 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -1259,7 +1259,7 @@
 ### Function : plat_get_ns_image_entrypoint() [mandatory]
 
     Argument : void
-    Return   : unsigned long
+    Return   : uintptr_t
 
 As previously described, BL2 is responsible for arranging for control to be
 passed to a normal world BL image through BL31. This function returns the